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This is m4.info, produced by makeinfo version 5.1 from m4.texi.
This manual (22 September 2013) is for GNU M4 (version 1.4.17), a
package containing an implementation of the m4 macro language.
Copyright (C) 1989-1994, 2004-2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts, and
no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License."
INFO-DIR-SECTION Text creation and manipulation
START-INFO-DIR-ENTRY
* M4: (m4). A powerful macro processor.
END-INFO-DIR-ENTRY

File: m4.info, Node: Top, Next: Preliminaries, Up: (dir)
GNU M4
******
This manual (22 September 2013) is for GNU M4 (version 1.4.17), a
package containing an implementation of the m4 macro language.
Copyright (C) 1989-1994, 2004-2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License,
Version 1.3 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts, and
no Back-Cover Texts. A copy of the license is included in the
section entitled "GNU Free Documentation License."
GNU 'm4' is an implementation of the traditional UNIX macro
processor. It is mostly SVR4 compatible, although it has some
extensions (for example, handling more than 9 positional parameters to
macros). 'm4' also has builtin functions for including files, running
shell commands, doing arithmetic, etc. Autoconf needs GNU 'm4' for
generating 'configure' scripts, but not for running them.
GNU 'm4' was originally written by Rene' Seindal, with subsequent
changes by Franc,ois Pinard and other volunteers on the Internet. All
names and email addresses can be found in the files 'm4-1.4.17/AUTHORS'
and 'm4-1.4.17/THANKS' from the GNU M4 distribution.
This is release 1.4.17. It is now considered stable: future releases
in the 1.4.x series are only meant to fix bugs, increase speed, or
improve documentation. However...
An experimental feature, which would improve 'm4' usefulness, allows
for changing the syntax for what is a "word" in 'm4'. You should use:
./configure --enable-changeword
if you want this feature compiled in. The current implementation slows
down 'm4' considerably and is hardly acceptable. In the future, 'm4'
2.0 will come with a different set of new features that provide similar
capabilities, but without the inefficiencies, so changeword will go away
and _you should not count on it_.
* Menu:
* Preliminaries:: Introduction and preliminaries
* Invoking m4:: Invoking 'm4'
* Syntax:: Lexical and syntactic conventions
* Macros:: How to invoke macros
* Definitions:: How to define new macros
* Conditionals:: Conditionals, loops, and recursion
* Debugging:: How to debug macros and input
* Input Control:: Input control
* File Inclusion:: File inclusion
* Diversions:: Diverting and undiverting output
* Text handling:: Macros for text handling
* Arithmetic:: Macros for doing arithmetic
* Shell commands:: Macros for running shell commands
* Miscellaneous:: Miscellaneous builtin macros
* Frozen files:: Fast loading of frozen state
* Compatibility:: Compatibility with other versions of 'm4'
* Answers:: Correct version of some examples
* Copying This Package:: How to make copies of the overall M4 package
* Copying This Manual:: How to make copies of this manual
* Indices:: Indices of concepts and macros
-- The Detailed Node Listing --
Introduction and preliminaries
* Intro:: Introduction to 'm4'
* History:: Historical references
* Bugs:: Problems and bugs
* Manual:: Using this manual
Invoking 'm4'
* Operation modes:: Command line options for operation modes
* Preprocessor features:: Command line options for preprocessor features
* Limits control:: Command line options for limits control
* Frozen state:: Command line options for frozen state
* Debugging options:: Command line options for debugging
* Command line files:: Specifying input files on the command line
Lexical and syntactic conventions
* Names:: Macro names
* Quoted strings:: Quoting input to 'm4'
* Comments:: Comments in 'm4' input
* Other tokens:: Other kinds of input tokens
* Input processing:: How 'm4' copies input to output
How to invoke macros
* Invocation:: Macro invocation
* Inhibiting Invocation:: Preventing macro invocation
* Macro Arguments:: Macro arguments
* Quoting Arguments:: On Quoting Arguments to macros
* Macro expansion:: Expanding macros
How to define new macros
* Define:: Defining a new macro
* Arguments:: Arguments to macros
* Pseudo Arguments:: Special arguments to macros
* Undefine:: Deleting a macro
* Defn:: Renaming macros
* Pushdef:: Temporarily redefining macros
* Indir:: Indirect call of macros
* Builtin:: Indirect call of builtins
Conditionals, loops, and recursion
* Ifdef:: Testing if a macro is defined
* Ifelse:: If-else construct, or multibranch
* Shift:: Recursion in 'm4'
* Forloop:: Iteration by counting
* Foreach:: Iteration by list contents
* Stacks:: Working with definition stacks
* Composition:: Building macros with macros
How to debug macros and input
* Dumpdef:: Displaying macro definitions
* Trace:: Tracing macro calls
* Debug Levels:: Controlling debugging output
* Debug Output:: Saving debugging output
Input control
* Dnl:: Deleting whitespace in input
* Changequote:: Changing the quote characters
* Changecom:: Changing the comment delimiters
* Changeword:: Changing the lexical structure of words
* M4wrap:: Saving text until end of input
File inclusion
* Include:: Including named files
* Search Path:: Searching for include files
Diverting and undiverting output
* Divert:: Diverting output
* Undivert:: Undiverting output
* Divnum:: Diversion numbers
* Cleardivert:: Discarding diverted text
Macros for text handling
* Len:: Calculating length of strings
* Index macro:: Searching for substrings
* Regexp:: Searching for regular expressions
* Substr:: Extracting substrings
* Translit:: Translating characters
* Patsubst:: Substituting text by regular expression
* Format:: Formatting strings (printf-like)
Macros for doing arithmetic
* Incr:: Decrement and increment operators
* Eval:: Evaluating integer expressions
Macros for running shell commands
* Platform macros:: Determining the platform
* Syscmd:: Executing simple commands
* Esyscmd:: Reading the output of commands
* Sysval:: Exit status
* Mkstemp:: Making temporary files
Miscellaneous builtin macros
* Errprint:: Printing error messages
* Location:: Printing current location
* M4exit:: Exiting from 'm4'
Fast loading of frozen state
* Using frozen files:: Using frozen files
* Frozen file format:: Frozen file format
Compatibility with other versions of 'm4'
* Extensions:: Extensions in GNU M4
* Incompatibilities:: Facilities in System V m4 not in GNU M4
* Other Incompatibilities:: Other incompatibilities
Correct version of some examples
* Improved exch:: Solution for 'exch'
* Improved forloop:: Solution for 'forloop'
* Improved foreach:: Solution for 'foreach'
* Improved copy:: Solution for 'copy'
* Improved m4wrap:: Solution for 'm4wrap'
* Improved cleardivert:: Solution for 'cleardivert'
* Improved capitalize:: Solution for 'capitalize'
* Improved fatal_error:: Solution for 'fatal_error'
How to make copies of the overall M4 package
* GNU General Public License:: License for copying the M4 package
How to make copies of this manual
* GNU Free Documentation License:: License for copying this manual
Indices of concepts and macros
* Macro index:: Index for all 'm4' macros
* Concept index:: Index for many concepts

File: m4.info, Node: Preliminaries, Next: Invoking m4, Prev: Top, Up: Top
1 Introduction and preliminaries
********************************
This first chapter explains what GNU 'm4' is, where 'm4' comes from, how
to read and use this documentation, how to call the 'm4' program, and
how to report bugs about it. It concludes by giving tips for reading
the remainder of the manual.
The following chapters then detail all the features of the 'm4'
language.
* Menu:
* Intro:: Introduction to 'm4'
* History:: Historical references
* Bugs:: Problems and bugs
* Manual:: Using this manual

File: m4.info, Node: Intro, Next: History, Up: Preliminaries
1.1 Introduction to 'm4'
========================
'm4' is a macro processor, in the sense that it copies its input to the
output, expanding macros as it goes. Macros are either builtin or
user-defined, and can take any number of arguments. Besides just doing
macro expansion, 'm4' has builtin functions for including named files,
running shell commands, doing integer arithmetic, manipulating text in
various ways, performing recursion, etc.... 'm4' can be used either as a
front-end to a compiler, or as a macro processor in its own right.
The 'm4' macro processor is widely available on all UNIXes, and has
been standardized by POSIX. Usually, only a small percentage of users
are aware of its existence. However, those who find it often become
committed users. The popularity of GNU Autoconf, which requires GNU
'm4' for _generating_ 'configure' scripts, is an incentive for many to
install it, while these people will not themselves program in 'm4'. GNU
'm4' is mostly compatible with the System V, Release 4 version, except
for some minor differences. *Note Compatibility::, for more details.
Some people find 'm4' to be fairly addictive. They first use 'm4'
for simple problems, then take bigger and bigger challenges, learning
how to write complex sets of 'm4' macros along the way. Once really
addicted, users pursue writing of sophisticated 'm4' applications even
to solve simple problems, devoting more time debugging their 'm4'
scripts than doing real work. Beware that 'm4' may be dangerous for the
health of compulsive programmers.

File: m4.info, Node: History, Next: Bugs, Prev: Intro, Up: Preliminaries
1.2 Historical references
=========================
Macro languages were invented early in the history of computing. In the
1950s Alan Perlis suggested that the macro language be independent of
the language being processed. Techniques such as conditional and
recursive macros, and using macros to define other macros, were
described by Doug McIlroy of Bell Labs in "Macro Instruction Extensions
of Compiler Languages", _Communications of the ACM_ 3, 4 (1960), 214-20,
<http://dx.doi.org/10.1145/367177.367223>.
An important precursor of 'm4' was GPM; see C. Strachey, "A general
purpose macrogenerator", _Computer Journal_ 8, 3 (1965), 225-41,
<http://dx.doi.org/10.1093/comjnl/8.3.225>. GPM is also succinctly
described in David Gries's book _Compiler Construction for Digital
Computers_, Wiley (1971). Strachey was a brilliant programmer: GPM fit
into 250 machine instructions!
Inspired by GPM while visiting Strachey's Lab in 1968, McIlroy wrote
a model preprocessor in that fit into a page of Snobol 3 code, and
McIlroy and Robert Morris developed a series of further models at Bell
Labs. Andrew D. Hall followed up with M6, a general purpose macro
processor used to port the Fortran source code of the Altran computer
algebra system; see Hall's "The M6 Macro Processor", Computing Science
Technical Report #2, Bell Labs (1972),
<http://cm.bell-labs.com/cm/cs/cstr/2.pdf>. M6's source code consisted
of about 600 Fortran statements. Its name was the first of the 'm4'
line.
The Brian Kernighan and P.J. Plauger book _Software Tools_,
Addison-Wesley (1976), describes and implements a Unix macro-processor
language, which inspired Dennis Ritchie to write 'm3', a macro processor
for the AP-3 minicomputer.
Kernighan and Ritchie then joined forces to develop the original
'm4', described in "The M4 Macro Processor", Bell Laboratories (1977),
<http://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf>. It had only
21 builtin macros.
While 'GPM' was more _pure_, 'm4' is meant to deal with the true
intricacies of real life: macros can be recognized without being
pre-announced, skipping whitespace or end-of-lines is easier, more
constructs are builtin instead of derived, etc.
Originally, the Kernighan and Plauger macro-processor, and then 'm3',
formed the engine for the Rational FORTRAN preprocessor, that is, the
'Ratfor' equivalent of 'cpp'. Later, 'm4' was used as a front-end for
'Ratfor', 'C' and 'Cobol'.
Rene' Seindal released his implementation of 'm4', GNU 'm4', in 1990,
with the aim of removing the artificial limitations in many of the
traditional 'm4' implementations, such as maximum line length, macro
size, or number of macros.
The late Professor A. Dain Samples described and implemented a
further evolution in the form of 'M5': "User's Guide to the M5 Macro
Language: 2nd edition", Electronic Announcement on comp.compilers
newsgroup (1992).
Franc,ois Pinard took over maintenance of GNU 'm4' in 1992, until
1994 when he released GNU 'm4' 1.4, which was the stable release for 10
years. It was at this time that GNU Autoconf decided to require GNU
'm4' as its underlying engine, since all other implementations of 'm4'
had too many limitations.
More recently, in 2004, Paul Eggert released 1.4.1 and 1.4.2 which
addressed some long standing bugs in the venerable 1.4 release. Then in
2005, Gary V. Vaughan collected together the many patches to GNU 'm4'
1.4 that were floating around the net and released 1.4.3 and 1.4.4. And
in 2006, Eric Blake joined the team and prepared patches for the release
of 1.4.5, 1.4.6, 1.4.7, and 1.4.8. More bug fixes were incorporated in
2007, with releases 1.4.9 and 1.4.10. Eric continued with some
portability fixes for 1.4.11 and 1.4.12 in 2008, 1.4.13 in 2009, 1.4.14
and 1.4.15 in 2010, and 1.4.16 in 2011.
Meanwhile, development has continued on new features for 'm4', such
as dynamic module loading and additional builtins. When complete, GNU
'm4' 2.0 will start a new series of releases.

File: m4.info, Node: Bugs, Next: Manual, Prev: History, Up: Preliminaries
1.3 Problems and bugs
=====================
If you have problems with GNU M4 or think you've found a bug, please
report it. Before reporting a bug, make sure you've actually found a
real bug. Carefully reread the documentation and see if it really says
you can do what you're trying to do. If it's not clear whether you
should be able to do something or not, report that too; it's a bug in
the documentation!
Before reporting a bug or trying to fix it yourself, try to isolate
it to the smallest possible input file that reproduces the problem.
Then send us the input file and the exact results 'm4' gave you. Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.
Once you've got a precise problem, send e-mail to <bug-m4@gnu.org>.
Please include the version number of 'm4' you are using. You can get
this information with the command 'm4 --version'. Also provide details
about the platform you are executing on.
Non-bug suggestions are always welcome as well. If you have
questions about things that are unclear in the documentation or are just
obscure features, please report them too.

File: m4.info, Node: Manual, Prev: Bugs, Up: Preliminaries
1.4 Using this manual
=====================
This manual contains a number of examples of 'm4' input and output, and
a simple notation is used to distinguish input, output and error
messages from 'm4'. Examples are set out from the normal text, and
shown in a fixed width font, like this
This is an example of an example!
To distinguish input from output, all output from 'm4' is prefixed by
the string '=>', and all error messages by the string 'error->'. When
showing how command line options affect matters, the command line is
shown with a prompt '$ 'like this'', otherwise, you can assume that a
simple 'm4' invocation will work. Thus:
$ command line to invoke m4
Example of input line
=>Output line from m4
error->and an error message
The sequence '^D' in an example indicates the end of the input file.
The sequence '<NL>' refers to the newline character. The majority of
these examples are self-contained, and you can run them with similar
results by invoking 'm4 -d'. In fact, the testsuite that is bundled in
the GNU M4 package consists of the examples in this document! Some of
the examples assume that your current directory is located where you
unpacked the installation, so if you plan on following along, you may
find it helpful to do this now:
$ cd m4-1.4.17
As each of the predefined macros in 'm4' is described, a prototype
call of the macro will be shown, giving descriptive names to the
arguments, e.g.,
-- Composite: example (STRING, [COUNT = '1']
[ARGUMENT]This is a sample prototype. There is not really a macro
named 'example', but this documents that if there were, it would be
a Composite macro, rather than a Builtin. It requires at least one
argument, STRING. Remember that in 'm4', there must not be a space
between the macro name and the opening parenthesis, unless it was
intended to call the macro without any arguments. The brackets
around COUNT and ARGUMENT show that these arguments are optional.
If COUNT is omitted, the macro behaves as if count were '1',
whereas if ARGUMENT is omitted, the macro behaves as if it were the
empty string. A blank argument is not the same as an omitted
argument. For example, 'example(`a')', 'example(`a',`1')', and
'example(`a',`1',)' would behave identically with COUNT set to '1';
while 'example(`a',)' and 'example(`a',`')' would explicitly pass
the empty string for COUNT. The ellipses ('...') show that the
macro processes additional arguments after ARGUMENT, rather than
ignoring them.
All macro arguments in 'm4' are strings, but some are given special
interpretation, e.g., as numbers, file names, regular expressions, etc.
The documentation for each macro will state how the parameters are
interpreted, and what happens if the argument cannot be parsed according
to the desired interpretation. Unless specified otherwise, a parameter
specified to be a number is parsed as a decimal, even if the argument
has leading zeros; and parsing the empty string as a number results in 0
rather than an error, although a warning will be issued.
This document consistently writes and uses "builtin", without a
hyphen, as if it were an English word. This is how the 'builtin'
primitive is spelled within 'm4'.

File: m4.info, Node: Invoking m4, Next: Syntax, Prev: Preliminaries, Up: Top
2 Invoking 'm4'
***************
The format of the 'm4' command is:
m4 [OPTION...] [FILE...]
All options begin with '-', or if long option names are used, with
'--'. A long option name need not be written completely, any
unambiguous prefix is sufficient. POSIX requires 'm4' to recognize
arguments intermixed with files, even when 'POSIXLY_CORRECT' is set in
the environment. Most options take effect at startup regardless of
their position, but some are documented below as taking effect after any
files that occurred earlier in the command line. The argument '--' is a
marker to denote the end of options.
With short options, options that do not take arguments may be
combined into a single command line argument with subsequent options,
options with mandatory arguments may be provided either as a single
command line argument or as two arguments, and options with optional
arguments must be provided as a single argument. In other words, 'm4
-QPDfoo -d a -df' is equivalent to 'm4 -Q -P -D foo -d -df -- ./a',
although the latter form is considered canonical.
With long options, options with mandatory arguments may be provided
with an equal sign ('=') in a single argument, or as two arguments, and
options with optional arguments must be provided as a single argument.
In other words, 'm4 --def foo --debug a' is equivalent to 'm4
--define=foo --debug= -- ./a', although the latter form is considered
canonical (not to mention more robust, in case a future version of 'm4'
introduces an option named '--default').
'm4' understands the following options, grouped by functionality.
* Menu:
* Operation modes:: Command line options for operation modes
* Preprocessor features:: Command line options for preprocessor features
* Limits control:: Command line options for limits control
* Frozen state:: Command line options for frozen state
* Debugging options:: Command line options for debugging
* Command line files:: Specifying input files on the command line

File: m4.info, Node: Operation modes, Next: Preprocessor features, Up: Invoking m4
2.1 Command line options for operation modes
============================================
Several options control the overall operation of 'm4':
'--help'
Print a help summary on standard output, then immediately exit 'm4'
without reading any input files or performing any other actions.
'--version'
Print the version number of the program on standard output, then
immediately exit 'm4' without reading any input files or performing
any other actions.
'-E'
'--fatal-warnings'
Controls the effect of warnings. If unspecified, then execution
continues and exit status is unaffected when a warning is printed.
If specified exactly once, warnings become fatal; when one is
issued, execution continues, but the exit status will be non-zero.
If specified multiple times, then execution halts with non-zero
status the first time a warning is issued. The introduction of
behavior levels is new to M4 1.4.9; for behavior consistent with
earlier versions, you should specify '-E' twice.
'-i'
'--interactive'
'-e'
Makes this invocation of 'm4' interactive. This means that all
output will be unbuffered, and interrupts will be ignored. The
spelling '-e' exists for compatibility with other 'm4'
implementations, and issues a warning because it may be withdrawn
in a future version of GNU M4.
'-P'
'--prefix-builtins'
Internally modify _all_ builtin macro names so they all start with
the prefix 'm4_'. For example, using this option, one should write
'm4_define' instead of 'define', and 'm4___file__' instead of
'__file__'. This option has no effect if '-R' is also specified.
'-Q'
'--quiet'
'--silent'
Suppress warnings, such as missing or superfluous arguments in
macro calls, or treating the empty string as zero.
'--warn-macro-sequence[=REGEXP]'
Issue a warning if the regular expression REGEXP has a non-empty
match in any macro definition (either by 'define' or 'pushdef').
Empty matches are ignored; therefore, supplying the empty string as
REGEXP disables any warning. If the optional REGEXP is not
supplied, then the default regular expression is
'\$\({[^}]*}\|[0-9][0-9]+\)' (a literal '$' followed by multiple
digits or by an open brace), since these sequences will change
semantics in the default operation of GNU M4 2.0 (due to a change
in how more than 9 arguments in a macro definition will be handled,
*note Arguments::). Providing an alternate regular expression can
provide a useful reverse lookup feature of finding where a macro is
defined to have a given definition.
'-W REGEXP'
'--word-regexp=REGEXP'
Use REGEXP as an alternative syntax for macro names. This
experimental option will not be present in all GNU 'm4'
implementations (*note Changeword::).

File: m4.info, Node: Preprocessor features, Next: Limits control, Prev: Operation modes, Up: Invoking m4
2.2 Command line options for preprocessor features
==================================================
Several options allow 'm4' to behave more like a preprocessor. Macro
definitions and deletions can be made on the command line, the search
path can be altered, and the output file can track where the input came
from. These features occur with the following options:
'-D NAME[=VALUE]'
'--define=NAME[=VALUE]'
This enters NAME into the symbol table. If '=VALUE' is missing,
the value is taken to be the empty string. The VALUE can be any
string, and the macro can be defined to take arguments, just as if
it was defined from within the input. This option may be given
more than once; order with respect to file names is significant,
and redefining the same NAME loses the previous value.
'-I DIRECTORY'
'--include=DIRECTORY'
Make 'm4' search DIRECTORY for included files that are not found in
the current working directory. *Note Search Path::, for more
details. This option may be given more than once.
'-s'
'--synclines'
Generate synchronization lines, for use by the C preprocessor or
other similar tools. Order is significant with respect to file
names. This option is useful, for example, when 'm4' is used as a
front end to a compiler. Source file name and line number
information is conveyed by directives of the form '#line LINENUM
"FILE"', which are inserted as needed into the middle of the
output. Such directives mean that the following line originated or
was expanded from the contents of input file FILE at line LINENUM.
The '"FILE"' part is often omitted when the file name did not
change from the previous directive.
Synchronization directives are always given on complete lines by
themselves. When a synchronization discrepancy occurs in the
middle of an output line, the associated synchronization directive
is delayed until the next newline that does not occur in the middle
of a quoted string or comment.
define(`twoline', `1
2')
=>#line 2 "stdin"
=>
changecom(`/*', `*/')
=>
define(`comment', `/*1
2*/')
=>#line 5
=>
dnl no line
hello
=>#line 7
=>hello
twoline
=>1
=>#line 8
=>2
comment
=>/*1
=>2*/
one comment `two
three'
=>#line 10
=>one /*1
=>2*/ two
=>three
goodbye
=>#line 12
=>goodbye
'-U NAME'
'--undefine=NAME'
This deletes any predefined meaning NAME might have. Obviously,
only predefined macros can be deleted in this way. This option may
be given more than once; undefining a NAME that does not have a
definition is silently ignored. Order is significant with respect
to file names.

File: m4.info, Node: Limits control, Next: Frozen state, Prev: Preprocessor features, Up: Invoking m4
2.3 Command line options for limits control
===========================================
There are some limits within 'm4' that can be tuned. For compatibility,
'm4' also accepts some options that control limits in other
implementations, but which are automatically unbounded (limited only by
your hardware and operating system constraints) in GNU 'm4'.
'-g'
'--gnu'
Enable all the extensions in this implementation. In this release
of M4, this option is always on by default; it is currently only
useful when overriding a prior use of '--traditional'. However,
having GNU behavior as default makes it impossible to write a
strictly POSIX-compliant client that avoids all incompatible GNU M4
extensions, since such a client would have to use the non-POSIX
command-line option to force full POSIX behavior. Thus, a future
version of M4 will be changed to implicitly use the option
'--traditional' if the environment variable 'POSIXLY_CORRECT' is
set. Projects that intentionally use GNU extensions should
consider using '--gnu' to state their intentions, so that the
project will not mysteriously break if the user upgrades to a newer
M4 and has 'POSIXLY_CORRECT' set in their environment.
'-G'
'--traditional'
Suppress all the extensions made in this implementation, compared
to the System V version. *Note Compatibility::, for a list of
these.
'-H NUM'
'--hashsize=NUM'
Make the internal hash table for symbol lookup be NUM entries big.
For better performance, the number should be prime, but this is not
checked. The default is 509 entries. It should not be necessary
to increase this value, unless you define an excessive number of
macros.
'-L NUM'
'--nesting-limit=NUM'
Artificially limit the nesting of macro calls to NUM levels,
stopping program execution if this limit is ever exceeded. When
not specified, nesting defaults to unlimited on platforms that can
detect stack overflow, and to 1024 levels otherwise. A value of
zero means unlimited; but then heavily nested code could
potentially cause a stack overflow.
The precise effect of this option is more correctly associated with
textual nesting than dynamic recursion. It has been useful when
some complex 'm4' input was generated by mechanical means, and also
in diagnosing recursive algorithms that do not scale well. Most
users never need to change this option from its default.
This option does _not_ have the ability to break endless rescanning
loops, since these do not necessarily consume much memory or stack
space. Through clever usage of rescanning loops, one can request
complex, time-consuming computations from 'm4' with useful results.
Putting limitations in this area would break 'm4' power. There are
many pathological cases: 'define(`a', `a')a' is only the simplest
example (but *note Compatibility::). Expecting GNU 'm4' to detect
these would be a little like expecting a compiler system to detect
and diagnose endless loops: it is a quite _hard_ problem in
general, if not undecidable!
'-B NUM'
'-S NUM'
'-T NUM'
These options are present for compatibility with System V 'm4', but
do nothing in this implementation. They may disappear in future
releases, and issue a warning to that effect.
'-N NUM'
'--diversions=NUM'
These options are present only for compatibility with previous
versions of GNU 'm4', and were controlling the number of possible
diversions which could be used at the same time. They do nothing,
because there is no fixed limit anymore. They may disappear in
future releases, and issue a warning to that effect.

File: m4.info, Node: Frozen state, Next: Debugging options, Prev: Limits control, Up: Invoking m4
2.4 Command line options for frozen state
=========================================
GNU 'm4' comes with a feature of freezing internal state (*note Frozen
files::). This can be used to speed up 'm4' execution when reusing a
common initialization script.
'-F FILE'
'--freeze-state=FILE'
Once execution is finished, write out the frozen state on the
specified FILE. It is conventional, but not required, for FILE to
end in '.m4f'.
'-R FILE'
'--reload-state=FILE'
Before execution starts, recover the internal state from the
specified frozen FILE. The options '-D', '-U', and '-t' take
effect after state is reloaded, but before the input files are
read.

File: m4.info, Node: Debugging options, Next: Command line files, Prev: Frozen state, Up: Invoking m4
2.5 Command line options for debugging
======================================
Finally, there are several options for aiding in debugging 'm4' scripts.
'-d[FLAGS]'
'--debug[=FLAGS]'
Set the debug-level according to the flags FLAGS. The debug-level
controls the format and amount of information presented by the
debugging functions. *Note Debug Levels::, for more details on the
format and meaning of FLAGS. If omitted, FLAGS defaults to 'aeq'.
'--debugfile[=FILE]'
'-o FILE'
'--error-output=FILE'
Redirect 'dumpdef' output, debug messages, and trace output to the
named FILE. Warnings, error messages, and 'errprint' output are
still printed to standard error. If these options are not used, or
if FILE is unspecified (only possible for '--debugfile'), debug
output goes to standard error; if FILE is the empty string, debug
output is discarded. *Note Debug Output::, for more details. The
option '--debugfile' may be given more than once, and order is
significant with respect to file names. The spellings '-o' and
'--error-output' are misleading and inconsistent with other GNU
tools; for now they are silently accepted as synonyms of
'--debugfile' and only recognized once, but in a future version of
M4, using them will cause a warning to be issued.
'-l NUM'
'--arglength=NUM'
Restrict the size of the output generated by macro tracing to NUM
characters per trace line. If unspecified or zero, output is
unlimited. *Note Debug Levels::, for more details.
'-t NAME'
'--trace=NAME'
This enables tracing for the macro NAME, at any point where it is
defined. NAME need not be defined when this option is given. This
option may be given more than once, and order is significant with
respect to file names. *Note Trace::, for more details.

File: m4.info, Node: Command line files, Prev: Debugging options, Up: Invoking m4
2.6 Specifying input files on the command line
==============================================
The remaining arguments on the command line are taken to be input file
names. If no names are present, standard input is read. A file name of
'-' is taken to mean standard input. It is conventional, but not
required, for input files to end in '.m4'.
The input files are read in the sequence given. Standard input can
be read more than once, so the file name '-' may appear multiple times
on the command line; this makes a difference when input is from a
terminal or other special file type. It is an error if an input file
ends in the middle of argument collection, a comment, or a quoted
string.
The options '--define' ('-D'), '--undefine' ('-U'), '--synclines'
('-s'), and '--trace' ('-t') only take effect after processing input
from any file names that occur earlier on the command line. For
example, assume the file 'foo' contains:
$ cat foo
bar
The text 'bar' can then be redefined over multiple uses of 'foo':
$ m4 -Dbar=hello foo -Dbar=world foo
=>hello
=>world
If none of the input files invoked 'm4exit' (*note M4exit::), the
exit status of 'm4' will be 0 for success, 1 for general failure (such
as problems with reading an input file), and 63 for version mismatch
(*note Using frozen files::).
If you need to read a file whose name starts with a '-', you can
specify it as './-file', or use '--' to mark the end of options.

File: m4.info, Node: Syntax, Next: Macros, Prev: Invoking m4, Up: Top
3 Lexical and syntactic conventions
***********************************
As 'm4' reads its input, it separates it into "tokens". A token is
either a name, a quoted string, or any single character, that is not a
part of either a name or a string. Input to 'm4' can also contain
comments. GNU 'm4' does not yet understand multibyte locales; all
operations are byte-oriented rather than character-oriented (although if
your locale uses a single byte encoding, such as ISO-8859-1, you will
not notice a difference). However, 'm4' is eight-bit clean, so you can
use non-ASCII characters in quoted strings (*note Changequote::),
comments (*note Changecom::), and macro names (*note Indir::), with the
exception of the NUL character (the zero byte ''\0'').
* Menu:
* Names:: Macro names
* Quoted strings:: Quoting input to 'm4'
* Comments:: Comments in 'm4' input
* Other tokens:: Other kinds of input tokens
* Input processing:: How 'm4' copies input to output

File: m4.info, Node: Names, Next: Quoted strings, Up: Syntax
3.1 Macro names
===============
A name is any sequence of letters, digits, and the character '_'
(underscore), where the first character is not a digit. 'm4' will use
the longest such sequence found in the input. If a name has a macro
definition, it will be subject to macro expansion (*note Macros::).
Names are case-sensitive.
Examples of legal names are: 'foo', '_tmp', and 'name01'.

File: m4.info, Node: Quoted strings, Next: Comments, Prev: Names, Up: Syntax
3.2 Quoting input to 'm4'
=========================
A quoted string is a sequence of characters surrounded by quote strings,
defaulting to '`' and ''', where the nested begin and end quotes within
the string are balanced. The value of a string token is the text, with
one level of quotes stripped off. Thus
`'
=>
is the empty string, and double-quoting turns into single-quoting.
``quoted''
=>`quoted'
The quote characters can be changed at any time, using the builtin
macro 'changequote'. *Note Changequote::, for more information.

File: m4.info, Node: Comments, Next: Other tokens, Prev: Quoted strings, Up: Syntax
3.3 Comments in 'm4' input
==========================
Comments in 'm4' are normally delimited by the characters '#' and
newline. All characters between the comment delimiters are ignored, but
the entire comment (including the delimiters) is passed through to the
output--comments are _not_ discarded by 'm4'.
Comments cannot be nested, so the first newline after a '#' ends the
comment. The commenting effect of the begin-comment string can be
inhibited by quoting it.
$ m4
`quoted text' # `commented text'
=>quoted text # `commented text'
`quoting inhibits' `#' `comments'
=>quoting inhibits # comments
The comment delimiters can be changed to any string at any time,
using the builtin macro 'changecom'. *Note Changecom::, for more
information.

File: m4.info, Node: Other tokens, Next: Input processing, Prev: Comments, Up: Syntax
3.4 Other kinds of input tokens
===============================
Any character, that is neither a part of a name, nor of a quoted string,
nor a comment, is a token by itself. When not in the context of macro
expansion, all of these tokens are just copied to output. However,
during macro expansion, whitespace characters (space, tab, newline,
formfeed, carriage return, vertical tab), parentheses ('(' and ')'),
comma (','), and dollar ('$') have additional roles, explained later.

File: m4.info, Node: Input processing, Prev: Other tokens, Up: Syntax
3.5 How 'm4' copies input to output
===================================
As 'm4' reads the input token by token, it will copy each token directly
to the output immediately.
The exception is when it finds a word with a macro definition. In
that case 'm4' will calculate the macro's expansion, possibly reading
more input to get the arguments. It then inserts the expansion in front
of the remaining input. In other words, the resulting text from a macro
call will be read and parsed into tokens again.
'm4' expands a macro as soon as possible. If it finds a macro call
when collecting the arguments to another, it will expand the second call
first. This process continues until there are no more macro calls to
expand and all the input has been consumed.
For a running example, examine how 'm4' handles this input:
format(`Result is %d', eval(`2**15'))
First, 'm4' sees that the token 'format' is a macro name, so it collects
the tokens '(', '`Result is %d'', ',', and ' ', before encountering
another potential macro. Sure enough, 'eval' is a macro name, so the
nested argument collection picks up '(', '`2**15'', and ')', invoking
the eval macro with the lone argument of '2**15'. The expansion of
'eval(2**15)' is '32768', which is then rescanned as the five tokens
'3', '2', '7', '6', and '8'; and combined with the next ')', the format
macro now has all its arguments, as if the user had typed:
format(`Result is %d', 32768)
The format macro expands to 'Result is 32768', and we have another round
of scanning for the tokens 'Result', ' ', 'is', ' ', '3', '2', '7', '6',
and '8'. None of these are macros, so the final output is
=>Result is 32768
As a more complicated example, we will contrast an actual code
example from the Gnulib project(1), showing both a buggy approach and
the desired results. The user desires to output a shell assignment
statement that takes its argument and turns it into a shell variable by
converting it to uppercase and prepending a prefix. The original
attempt looks like this:
changequote([,])dnl
define([gl_STRING_MODULE_INDICATOR],
[
dnl comment
GNULIB_]translit([$1],[a-z],[A-Z])[=1
])dnl
gl_STRING_MODULE_INDICATOR([strcase])
=>
=> GNULIB_strcase=1
=>
Oops - the argument did not get capitalized. And although the manual
is not able to easily show it, both lines that appear empty actually
contain two trailing spaces. By stepping through the parse, it is easy
to see what happened. First, 'm4' sees the token 'changequote', which
it recognizes as a macro, followed by '(', '[', ',', ']', and ')' to
form the argument list. The macro expands to the empty string, but
changes the quoting characters to something more useful for generating
shell code (unbalanced '`' and ''' appear all the time in shell scripts,
but unbalanced '[]' tend to be rare). Also in the first line, 'm4' sees
the token 'dnl', which it recognizes as a builtin macro that consumes
the rest of the line, resulting in no output for that line.
The second line starts a macro definition. 'm4' sees the token
'define', which it recognizes as a macro, followed by a '(',
'[gl_STRING_MODULE_INDICATOR]', and ','. Because an unquoted comma was
encountered, the first argument is known to be the expansion of the
single-quoted string token, or 'gl_STRING_MODULE_INDICATOR'. Next, 'm4'
sees '<NL>', ' ', and ' ', but this whitespace is discarded as part of
argument collection. Then comes a rather lengthy single-quoted string
token, '[<NL> dnl comment<NL> GNULIB_]'. This is followed by the
token 'translit', which 'm4' recognizes as a macro name, so a nested
macro expansion has started.
The arguments to the 'translit' are found by the tokens '(', '[$1]',
',', '[a-z]', ',', '[A-Z]', and finally ')'. All three string arguments
are expanded (or in other words, the quotes are stripped), and since
neither '$' nor '1' need capitalization, the result of the macro is
'$1'. This expansion is rescanned, resulting in the two literal
characters '$' and '1'.
Scanning of the outer macro resumes, and picks up with '[=1<NL> ]',
and finally ')'. The collected pieces of expanded text are
concatenated, with the end result that the macro
'gl_STRING_MODULE_INDICATOR' is now defined to be the sequence
'<NL> dnl comment<NL> GNULIB_$1=1<NL> '. Once again, 'dnl' is
recognized and avoids a newline in the output.
The final line is then parsed, beginning with ' ' and ' ' that are
output literally. Then 'gl_STRING_MODULE_INDICATOR' is recognized as a
macro name, with an argument list of '(', '[strcase]', and ')'. Since
the definition of the macro contains the sequence '$1', that sequence is
replaced with the argument 'strcase' prior to starting the rescan. The
rescan sees '<NL>' and four spaces, which are output literally, then
'dnl', which discards the text ' comment<NL>'. Next comes four more
spaces, also output literally, and the token 'GNULIB_strcase', which
resulted from the earlier parameter substitution. Since that is not a
macro name, it is output literally, followed by the literal tokens '=',
'1', '<NL>', and two more spaces. Finally, the original '<NL>' seen
after the macro invocation is scanned and output literally.
Now for a corrected approach. This rearranges the use of newlines
and whitespace so that less whitespace is output (which, although
harmless to shell scripts, can be visually unappealing), and fixes the
quoting issues so that the capitalization occurs when the macro
'gl_STRING_MODULE_INDICATOR' is invoked, rather then when it is defined.
It also adds another layer of quoting to the first argument of
'translit', to ensure that the output will be rescanned as a string
rather than a potential uppercase macro name needing further expansion.
changequote([,])dnl
define([gl_STRING_MODULE_INDICATOR],
[dnl comment
GNULIB_[]translit([[$1]], [a-z], [A-Z])=1dnl
])dnl
gl_STRING_MODULE_INDICATOR([strcase])
=> GNULIB_STRCASE=1
The parsing of the first line is unchanged. The second line sees the
name of the macro to define, then sees the discarded '<NL>' and two
spaces, as before. But this time, the next token is '[dnl
comment<NL> GNULIB_[]translit([[$1]], [a-z], [A-Z])=1dnl<NL>]', which
includes nested quotes, followed by ')' to end the macro definition and
'dnl' to skip the newline. No early expansion of 'translit' occurs, so
the entire string becomes the definition of the macro.
The final line is then parsed, beginning with two spaces that are
output literally, and an invocation of 'gl_STRING_MODULE_INDICATOR' with
the argument 'strcase'. Again, the '$1' in the macro definition is
substituted prior to rescanning. Rescanning first encounters 'dnl', and
discards ' comment<NL>'. Then two spaces are output literally. Next
comes the token 'GNULIB_', but that is not a macro, so it is output
literally. The token '[]' is an empty string, so it does not affect
output. Then the token 'translit' is encountered.
This time, the arguments to 'translit' are parsed as '(',
'[[strcase]]', ',', ' ', '[a-z]', ',', ' ', '[A-Z]', and ')'. The two
spaces are discarded, and the translit results in the desired result
'[STRCASE]'. This is rescanned, but since it is a string, the quotes
are stripped and the only output is a literal 'STRCASE'. Then the
scanner sees '=' and '1', which are output literally, followed by 'dnl'
which discards the rest of the definition of
'gl_STRING_MODULE_INDICATOR'. The newline at the end of output is the
literal '<NL>' that appeared after the invocation of the macro.
The order in which 'm4' expands the macros can be further explored
using the trace facilities of GNU 'm4' (*note Trace::).
---------- Footnotes ----------
(1) Derived from a patch in
<http://lists.gnu.org/archive/html/bug-gnulib/2007-01/msg00389.html>,
and a followup patch in
<http://lists.gnu.org/archive/html/bug-gnulib/2007-02/msg00000.html>

File: m4.info, Node: Macros, Next: Definitions, Prev: Syntax, Up: Top
4 How to invoke macros
**********************
This chapter covers macro invocation, macro arguments and how macro
expansion is treated.
* Menu:
* Invocation:: Macro invocation
* Inhibiting Invocation:: Preventing macro invocation
* Macro Arguments:: Macro arguments
* Quoting Arguments:: On Quoting Arguments to macros
* Macro expansion:: Expanding macros

File: m4.info, Node: Invocation, Next: Inhibiting Invocation, Up: Macros
4.1 Macro invocation
====================
Macro invocations has one of the forms
name
which is a macro invocation without any arguments, or
name(arg1, arg2, ..., argN)
which is a macro invocation with N arguments. Macros can have any
number of arguments. All arguments are strings, but different macros
might interpret the arguments in different ways.
The opening parenthesis _must_ follow the NAME directly, with no
spaces in between. If it does not, the macro is called with no
arguments at all.
For a macro call to have no arguments, the parentheses _must_ be left
out. The macro call
name()
is a macro call with one argument, which is the empty string, not a call
with no arguments.

File: m4.info, Node: Inhibiting Invocation, Next: Macro Arguments, Prev: Invocation, Up: Macros
4.2 Preventing macro invocation
===============================
An innovation of the 'm4' language, compared to some of its predecessors
(like Strachey's 'GPM', for example), is the ability to recognize macro
calls without resorting to any special, prefixed invocation character.
While generally useful, this feature might sometimes be the source of
spurious, unwanted macro calls. So, GNU 'm4' offers several mechanisms
or techniques for inhibiting the recognition of names as macro calls.
First of all, many builtin macros cannot meaningfully be called
without arguments. As a GNU extension, for any of these macros,
whenever an opening parenthesis does not immediately follow their name,
the builtin macro call is not triggered. This solves the most usual
cases, like for 'include' or 'eval'. Later in this document, the
sentence "This macro is recognized only with parameters" refers to this
specific provision of GNU M4, also known as a blind builtin macro. For
the builtins defined by POSIX that bear this disclaimer, POSIX
specifically states that invoking those builtins without arguments is
unspecified, because many other implementations simply invoke the
builtin as though it were given one empty argument instead.
$ m4
eval
=>eval
eval(`1')
=>1
There is also a command line option ('--prefix-builtins', or '-P',
*note Invoking m4: Operation modes.) that renames all builtin macros
with a prefix of 'm4_' at startup. The option has no effect whatsoever
on user defined macros. For example, with this option, one has to write
'm4_dnl' and even 'm4_m4exit'. It also has no effect on whether a macro
requires parameters.
$ m4 -P
eval
=>eval
eval(`1')
=>eval(1)
m4_eval
=>m4_eval
m4_eval(`1')
=>1
Another alternative is to redefine problematic macros to a name less
likely to cause conflicts, using *note Definitions::.
If your version of GNU 'm4' has the 'changeword' feature compiled in,
it offers far more flexibility in specifying the syntax of macro names,
both builtin or user-defined. *Note Changeword::, for more information
on this experimental feature.
Of course, the simplest way to prevent a name from being interpreted
as a call to an existing macro is to quote it. The remainder of this
section studies a little more deeply how quoting affects macro
invocation, and how quoting can be used to inhibit macro invocation.
Even if quoting is usually done over the whole macro name, it can
also be done over only a few characters of this name (provided, of
course, that the unquoted portions are not also a macro). It is also
possible to quote the empty string, but this works only _inside_ the
name. For example:
`divert'
=>divert
`d'ivert
=>divert
di`ver't
=>divert
div`'ert
=>divert
all yield the string 'divert'. While in both:
`'divert
=>
divert`'
=>
the 'divert' builtin macro will be called, which expands to the empty
string.
The output of macro evaluations is always rescanned. In the
following example, the input 'x`'y' yields the string 'bCD', exactly as
if 'm4' has been given 'substr(ab`'cde, `1', `3')' as input:
define(`cde', `CDE')
=>
define(`x', `substr(ab')
=>
define(`y', `cde, `1', `3')')
=>
x`'y
=>bCD
Unquoted strings on either side of a quoted string are subject to
being recognized as macro names. In the following example, quoting the
empty string allows for the second 'macro' to be recognized as such:
define(`macro', `m')
=>
macro(`m')macro
=>mmacro
macro(`m')`'macro
=>mm
Quoting may prevent recognizing as a macro name the concatenation of
a macro expansion with the surrounding characters. In this example:
define(`macro', `di$1')
=>
macro(`v')`ert'
=>divert
macro(`v')ert
=>
the input will produce the string 'divert'. When the quotes were
removed, the 'divert' builtin was called instead.

File: m4.info, Node: Macro Arguments, Next: Quoting Arguments, Prev: Inhibiting Invocation, Up: Macros
4.3 Macro arguments
===================
When a name is seen, and it has a macro definition, it will be expanded
as a macro.
If the name is followed by an opening parenthesis, the arguments will
be collected before the macro is called. If too few arguments are
supplied, the missing arguments are taken to be the empty string.
However, some builtins are documented to behave differently for a
missing optional argument than for an explicit empty string. If there
are too many arguments, the excess arguments are ignored. Unquoted
leading whitespace is stripped off all arguments, but whitespace
generated by a macro expansion or occurring after a macro that expanded
to an empty string remains intact. Whitespace includes space, tab,
newline, carriage return, vertical tab, and formfeed.
define(`macro', `$1')
=>
macro( unquoted leading space lost)
=>unquoted leading space lost
macro(` quoted leading space kept')
=> quoted leading space kept
macro(
divert `unquoted space kept after expansion')
=> unquoted space kept after expansion
macro(macro(`
')`whitespace from expansion kept')
=>
=>whitespace from expansion kept
macro(`unquoted trailing whitespace kept'
)
=>unquoted trailing whitespace kept
=>
Normally 'm4' will issue warnings if a builtin macro is called with
an inappropriate number of arguments, but it can be suppressed with the
'--quiet' command line option (or '--silent', or '-Q', *note Invoking
m4: Operation modes.). For user defined macros, there is no check of
the number of arguments given.
$ m4
index(`abc')
error->m4:stdin:1: Warning: too few arguments to builtin `index'
=>0
index(`abc',)
=>0
index(`abc', `b', `ignored')
error->m4:stdin:3: Warning: excess arguments to builtin `index' ignored
=>1
$ m4 -Q
index(`abc')
=>0
index(`abc',)
=>0
index(`abc', `b', `ignored')
=>1
Macros are expanded normally during argument collection, and whatever
commas, quotes and parentheses that might show up in the resulting
expanded text will serve to define the arguments as well. Thus, if FOO
expands to ', b, c', the macro call
bar(a foo, d)
is a macro call with four arguments, which are 'a ', 'b', 'c' and 'd'.
To understand why the first argument contains whitespace, remember that
unquoted leading whitespace is never part of an argument, but trailing
whitespace always is.
It is possible for a macro's definition to change during argument
collection, in which case the expansion uses the definition that was in
effect at the time the opening '(' was seen.
define(`f', `1')
=>
f(define(`f', `2'))
=>1
f
=>2
It is an error if the end of file occurs while collecting arguments.
hello world
=>hello world
define(
^D
error->m4:stdin:2: ERROR: end of file in argument list

File: m4.info, Node: Quoting Arguments, Next: Macro expansion, Prev: Macro Arguments, Up: Macros
4.4 On Quoting Arguments to macros
==================================
Each argument has unquoted leading whitespace removed. Within each
argument, all unquoted parentheses must match. For example, if FOO is a
macro,
foo(() (`(') `(')
is a macro call, with one argument, whose value is '() (() ('. Commas
separate arguments, except when they occur inside quotes, comments, or
unquoted parentheses. *Note Pseudo Arguments::, for examples.
It is common practice to quote all arguments to macros, unless you
are sure you want the arguments expanded. Thus, in the above example
with the parentheses, the 'right' way to do it is like this:
foo(`() (() (')
It is, however, in certain cases necessary (because nested expansion
must occur to create the arguments for the outer macro) or convenient
(because it uses fewer characters) to leave out quotes for some
arguments, and there is nothing wrong in doing it. It just makes life a
bit harder, if you are not careful to follow a consistent quoting style.
For consistency, this manual follows the rule of thumb that each layer
of parentheses introduces another layer of single quoting, except when
showing the consequences of quoting rules. This is done even when the
quoted string cannot be a macro, such as with integers when you have not
changed the syntax via 'changeword' (*note Changeword::).
The quoting rule of thumb of one level of quoting per parentheses has
a nice property: when a macro name appears inside parentheses, you can
determine when it will be expanded. If it is not quoted, it will be
expanded prior to the outer macro, so that its expansion becomes the
argument. If it is single-quoted, it will be expanded after the outer
macro. And if it is double-quoted, it will be used as literal text
instead of a macro name.
define(`active', `ACT, IVE')
=>
define(`show', `$1 $1')
=>
show(active)
=>ACT ACT
show(`active')
=>ACT, IVE ACT, IVE
show(``active'')
=>active active

File: m4.info, Node: Macro expansion, Prev: Quoting Arguments, Up: Macros
4.5 Macro expansion
===================
When the arguments, if any, to a macro call have been collected, the
macro is expanded, and the expansion text is pushed back onto the input
(unquoted), and reread. The expansion text from one macro call might
therefore result in more macros being called, if the calls are included,
completely or partially, in the first macro calls' expansion.
Taking a very simple example, if FOO expands to 'bar', and BAR
expands to 'Hello', the input
$ m4 -Dbar=Hello -Dfoo=bar
foo
=>Hello
will expand first to 'bar', and when this is reread and expanded, into
'Hello'.

File: m4.info, Node: Definitions, Next: Conditionals, Prev: Macros, Up: Top
5 How to define new macros
**************************
Macros can be defined, redefined and deleted in several different ways.
Also, it is possible to redefine a macro without losing a previous
value, and bring back the original value at a later time.
* Menu:
* Define:: Defining a new macro
* Arguments:: Arguments to macros
* Pseudo Arguments:: Special arguments to macros
* Undefine:: Deleting a macro
* Defn:: Renaming macros
* Pushdef:: Temporarily redefining macros
* Indir:: Indirect call of macros
* Builtin:: Indirect call of builtins

File: m4.info, Node: Define, Next: Arguments, Up: Definitions
5.1 Defining a macro
====================
The normal way to define or redefine macros is to use the builtin
'define':
-- Builtin: define (NAME, [EXPANSION]
Defines NAME to expand to EXPANSION. If EXPANSION is not given, it
is taken to be empty.
The expansion of 'define' is void. The macro 'define' is
recognized only with parameters.
The following example defines the macro FOO to expand to the text
'Hello World.'.
define(`foo', `Hello world.')
=>
foo
=>Hello world.
The empty line in the output is there because the newline is not a
part of the macro definition, and it is consequently copied to the
output. This can be avoided by use of the macro 'dnl'. *Note Dnl::,
for details.
The first argument to 'define' should be quoted; otherwise, if the
macro is already defined, you will be defining a different macro. This
example shows the problems with underquoting, since we did not want to
redefine 'one':
define(foo, one)
=>
define(foo, two)
=>
one
=>two
GNU 'm4' normally replaces only the _topmost_ definition of a macro
if it has several definitions from 'pushdef' (*note Pushdef::). Some
other implementations of 'm4' replace all definitions of a macro with
'define'. *Note Incompatibilities::, for more details.
As a GNU extension, the first argument to 'define' does not have to
be a simple word. It can be any text string, even the empty string. A
macro with a non-standard name cannot be invoked in the normal way, as
the name is not recognized. It can only be referenced by the builtins
'indir' (*note Indir::) and 'defn' (*note Defn::).
Arrays and associative arrays can be simulated by using non-standard
macro names.
-- Composite: array (INDEX)
-- Composite: array_set (INDEX, [VALUE]
Provide access to entries within an array. 'array' reads the entry
at location INDEX, and 'array_set' assigns VALUE to location INDEX.
define(`array', `defn(format(``array[%d]'', `$1'))')
=>
define(`array_set', `define(format(``array[%d]'', `$1'), `$2')')
=>
array_set(`4', `array element no. 4')
=>
array_set(`17', `array element no. 17')
=>
array(`4')
=>array element no. 4
array(eval(`10 + 7'))
=>array element no. 17
Change the '%d' to '%s' and it is an associative array.

File: m4.info, Node: Arguments, Next: Pseudo Arguments, Prev: Define, Up: Definitions
5.2 Arguments to macros
=======================
Macros can have arguments. The Nth argument is denoted by '$n' in the
expansion text, and is replaced by the Nth actual argument, when the
macro is expanded. Replacement of arguments happens before rescanning,
regardless of how many nesting levels of quoting appear in the
expansion. Here is an example of a macro with two arguments.
-- Composite: exch (ARG1, ARG2)
Expands to ARG2 followed by ARG1, effectively exchanging their
order.
define(`exch', `$2, $1')
=>
exch(`arg1', `arg2')
=>arg2, arg1
This can be used, for example, if you like the arguments to 'define'
to be reversed.
define(`exch', `$2, $1')
=>
define(exch(``expansion text'', ``macro''))
=>
macro
=>expansion text
*Note Quoting Arguments::, for an explanation of the double quotes.
(You should try and improve this example so that clients of 'exch' do
not have to double quote; or *note Answers: Improved exch.).
As a special case, the zeroth argument, '$0', is always the name of
the macro being expanded.
define(`test', ``Macro name: $0'')
=>
test
=>Macro name: test
If you want quoted text to appear as part of the expansion text,
remember that quotes can be nested in quoted strings. Thus, in
define(`foo', `This is macro `foo'.')
=>
foo
=>This is macro foo.
The 'foo' in the expansion text is _not_ expanded, since it is a quoted
string, and not a name.
GNU 'm4' allows the number following the '$' to consist of one or
more digits, allowing macros to have any number of arguments. The
extension of accepting multiple digits is incompatible with POSIX, and
is different than traditional implementations of 'm4', which only
recognize one digit. Therefore, future versions of GNU M4 will phase
out this feature. To portably access beyond the ninth argument, you can
use the 'argn' macro documented later (*note Shift::).
POSIX also states that '$' followed immediately by '{' in a macro
definition is implementation-defined. This version of M4 passes the
literal characters '${' through unchanged, but M4 2.0 will implement an
optional feature similar to 'sh', where '${11}' expands to the eleventh
argument, to replace the current recognition of '$11'. Meanwhile, if
you want to guarantee that you will get a literal '${' in output when
expanding a macro, even when you upgrade to M4 2.0, you can use nested
quoting to your advantage:
define(`foo', `single quoted $`'{1} output')
=>
define(`bar', ``double quoted $'`{2} output'')
=>
foo(`a', `b')
=>single quoted ${1} output
bar(`a', `b')
=>double quoted ${2} output
To help you detect places in your M4 input files that might change in
behavior due to the changed behavior of M4 2.0, you can use the
'--warn-macro-sequence' command-line option (*note Invoking m4:
Operation modes.) with the default regular expression. This will add a
warning any time a macro definition includes '$' followed by multiple
digits, or by '{'. The warning is not enabled by default, because it
triggers a number of warnings in Autoconf 2.61 (and Autoconf uses '-E'
to treat warnings as errors), and because it will still be possible to
restore older behavior in M4 2.0.
$ m4 --warn-macro-sequence
define(`foo', `$001 ${1} $1')
error->m4:stdin:1: Warning: definition of `foo' contains sequence `$001'
error->m4:stdin:1: Warning: definition of `foo' contains sequence `${1}'
=>
foo(`bar')
=>bar ${1} bar

File: m4.info, Node: Pseudo Arguments, Next: Undefine, Prev: Arguments, Up: Definitions
5.3 Special arguments to macros
===============================
There is a special notation for the number of actual arguments supplied,
and for all the actual arguments.
The number of actual arguments in a macro call is denoted by '$#' in
the expansion text.
-- Composite: nargs (...)
Expands to a count of the number of arguments supplied.
define(`nargs', `$#')
=>
nargs
=>0
nargs()
=>1
nargs(`arg1', `arg2', `arg3')
=>3
nargs(`commas can be quoted, like this')
=>1
nargs(arg1#inside comments, commas do not separate arguments
still arg1)
=>1
nargs((unquoted parentheses, like this, group arguments))
=>1
Remember that '#' defaults to the comment character; if you forget
quotes to inhibit the comment behavior, your macro definition may not
end where you expected.
dnl Attempt to define a macro to just `$#'
define(underquoted, $#)
oops)
=>
underquoted
=>0)
=>oops
The notation '$*' can be used in the expansion text to denote all the
actual arguments, unquoted, with commas in between. For example
define(`echo', `$*')
=>
echo(arg1, arg2, arg3 , arg4)
=>arg1,arg2,arg3 ,arg4
Often each argument should be quoted, and the notation '$@' handles
that. It is just like '$*', except that it quotes each argument. A
simple example of that is:
define(`echo', `$@')
=>
echo(arg1, arg2, arg3 , arg4)
=>arg1,arg2,arg3 ,arg4
Where did the quotes go? Of course, they were eaten, when the
expanded text were reread by 'm4'. To show the difference, try
define(`echo1', `$*')
=>
define(`echo2', `$@')
=>
define(`foo', `This is macro `foo'.')
=>
echo1(foo)
=>This is macro This is macro foo..
echo1(`foo')
=>This is macro foo.
echo2(foo)
=>This is macro foo.
echo2(`foo')
=>foo
*Note Trace::, if you do not understand this. As another example of the
difference, remember that comments encountered in arguments are passed
untouched to the macro, and that quoting disables comments.
define(`echo1', `$*')
=>
define(`echo2', `$@')
=>
define(`foo', `bar')
=>
echo1(#foo'foo
foo)
=>#foo'foo
=>bar
echo2(#foo'foo
foo)
=>#foobar
=>bar'
A '$' sign in the expansion text, that is not followed by anything
'm4' understands, is simply copied to the macro expansion, as any other
text is.
define(`foo', `$$$ hello $$$')
=>
foo
=>$$$ hello $$$
If you want a macro to expand to something like '$12', the judicious
use of nested quoting can put a safe character between the '$' and the
next character, relying on the rescanning to remove the nested quote.
This will prevent 'm4' from interpreting the '$' sign as a reference to
an argument.
define(`foo', `no nested quote: $1')
=>
foo(`arg')
=>no nested quote: arg
define(`foo', `nested quote around $: `$'1')
=>
foo(`arg')
=>nested quote around $: $1
define(`foo', `nested empty quote after $: $`'1')
=>
foo(`arg')
=>nested empty quote after $: $1
define(`foo', `nested quote around next character: $`1'')
=>
foo(`arg')
=>nested quote around next character: $1
define(`foo', `nested quote around both: `$1'')
=>
foo(`arg')
=>nested quote around both: arg

File: m4.info, Node: Undefine, Next: Defn, Prev: Pseudo Arguments, Up: Definitions
5.4 Deleting a macro
====================
A macro definition can be removed with 'undefine':
-- Builtin: undefine (NAME...)
For each argument, remove the macro NAME. The macro names must
necessarily be quoted, since they will be expanded otherwise.
The expansion of 'undefine' is void. The macro 'undefine' is
recognized only with parameters.
foo bar blah
=>foo bar blah
define(`foo', `some')define(`bar', `other')define(`blah', `text')
=>
foo bar blah
=>some other text
undefine(`foo')
=>
foo bar blah
=>foo other text
undefine(`bar', `blah')
=>
foo bar blah
=>foo bar blah
Undefining a macro inside that macro's expansion is safe; the macro
still expands to the definition that was in effect at the '('.
define(`f', ``$0':$1')
=>
f(f(f(undefine(`f')`hello world')))
=>f:f:f:hello world
f(`bye')
=>f(bye)
It is not an error for NAME to have no macro definition. In that
case, 'undefine' does nothing.

File: m4.info, Node: Defn, Next: Pushdef, Prev: Undefine, Up: Definitions
5.5 Renaming macros
===================
It is possible to rename an already defined macro. To do this, you need
the builtin 'defn':
-- Builtin: defn (NAME...)
Expands to the _quoted definition_ of each NAME. If an argument is
not a defined macro, the expansion for that argument is empty.
If NAME is a user-defined macro, the quoted definition is simply
the quoted expansion text. If, instead, there is only one NAME and
it is a builtin, the expansion is a special token, which points to
the builtin's internal definition. This token is only meaningful
as the second argument to 'define' (and 'pushdef'), and is silently
converted to an empty string in most other contexts. Combining a
builtin with anything else is not supported; a warning is issued
and the builtin is omitted from the final expansion.
The macro 'defn' is recognized only with parameters.
Its normal use is best understood through an example, which shows how
to rename 'undefine' to 'zap':
define(`zap', defn(`undefine'))
=>
zap(`undefine')
=>
undefine(`zap')
=>undefine(zap)
In this way, 'defn' can be used to copy macro definitions, and also
definitions of builtin macros. Even if the original macro is removed,
the other name can still be used to access the definition.
The fact that macro definitions can be transferred also explains why
you should use '$0', rather than retyping a macro's name in its
definition:
define(`foo', `This is `$0'')
=>
define(`bar', defn(`foo'))
=>
bar
=>This is bar
Macros used as string variables should be referred through 'defn', to
avoid unwanted expansion of the text:
define(`string', `The macro dnl is very useful
')
=>
string
=>The macro
defn(`string')
=>The macro dnl is very useful
=>
However, it is important to remember that 'm4' rescanning is purely
textual. If an unbalanced end-quote string occurs in a macro
definition, the rescan will see that embedded quote as the termination
of the quoted string, and the remainder of the macro's definition will
be rescanned unquoted. Thus it is a good idea to avoid unbalanced
end-quotes in macro definitions or arguments to macros.
define(`foo', a'a)
=>
define(`a', `A')
=>
define(`echo', `$@')
=>
foo
=>A'A
defn(`foo')
=>aA'
echo(foo)
=>AA'
On the other hand, it is possible to exploit the fact that 'defn' can
concatenate multiple macros prior to the rescanning phase, in order to
join the definitions of macros that, in isolation, have unbalanced
quotes. This is particularly useful when one has used several macros to
accumulate text that M4 should rescan as a whole. In the example below,
note how the use of 'defn' on 'l' in isolation opens a string, which is
not closed until the next line; but used on 'l' and 'r' together results
in nested quoting.
define(`l', `<[>')define(`r', `<]>')
=>
changequote(`[', `]')
=>
defn([l])defn([r])
])
=><[>]defn([r])
=>)
defn([l], [r])
=><[>][<]>
Using 'defn' to generate special tokens for builtin macros outside of
expected contexts can sometimes trigger warnings. But most of the time,
such tokens are silently converted to the empty string.
$ m4 -d
defn(`defn')
=>
define(defn(`divnum'), `cannot redefine a builtin token')
error->m4:stdin:2: Warning: define: invalid macro name ignored
=>
divnum
=>0
len(defn(`divnum'))
=>0
Also note that 'defn' with multiple arguments can only join text
macros, not builtins, although a future version of GNU M4 may lift this
restriction.
$ m4 -d
define(`a', `A')define(`AA', `b')
=>
traceon(`defn', `define')
=>
defn(`a', `divnum', `a')
error->m4:stdin:3: Warning: cannot concatenate builtin `divnum'
error->m4trace: -1- defn(`a', `divnum', `a') -> ``A'`A''
=>AA
define(`mydivnum', defn(`divnum', `divnum'))mydivnum
error->m4:stdin:4: Warning: cannot concatenate builtin `divnum'
error->m4:stdin:4: Warning: cannot concatenate builtin `divnum'
error->m4trace: -2- defn(`divnum', `divnum')
error->m4trace: -1- define(`mydivnum', `')
=>
traceoff(`defn', `define')
=>

File: m4.info, Node: Pushdef, Next: Indir, Prev: Defn, Up: Definitions
5.6 Temporarily redefining macros
=================================
It is possible to redefine a macro temporarily, reverting to the
previous definition at a later time. This is done with the builtins
'pushdef' and 'popdef':
-- Builtin: pushdef (NAME, [EXPANSION]
-- Builtin: popdef (NAME...)
Analogous to 'define' and 'undefine'.
These macros work in a stack-like fashion. A macro is temporarily
redefined with 'pushdef', which replaces an existing definition of
NAME, while saving the previous definition, before the new one is
installed. If there is no previous definition, 'pushdef' behaves
exactly like 'define'.
If a macro has several definitions (of which only one is
accessible), the topmost definition can be removed with 'popdef'.
If there is no previous definition, 'popdef' behaves like
'undefine'.
The expansion of both 'pushdef' and 'popdef' is void. The macros
'pushdef' and 'popdef' are recognized only with parameters.
define(`foo', `Expansion one.')
=>
foo
=>Expansion one.
pushdef(`foo', `Expansion two.')
=>
foo
=>Expansion two.
pushdef(`foo', `Expansion three.')
=>
pushdef(`foo', `Expansion four.')
=>
popdef(`foo')
=>
foo
=>Expansion three.
popdef(`foo', `foo')
=>
foo
=>Expansion one.
popdef(`foo')
=>
foo
=>foo
If a macro with several definitions is redefined with 'define', the
topmost definition is _replaced_ with the new definition. If it is
removed with 'undefine', _all_ the definitions are removed, and not only
the topmost one. However, POSIX allows other implementations that treat
'define' as replacing an entire stack of definitions with a single new
definition, so to be portable to other implementations, it may be worth
explicitly using 'popdef' and 'pushdef' rather than relying on the GNU
behavior of 'define'.
define(`foo', `Expansion one.')
=>
foo
=>Expansion one.
pushdef(`foo', `Expansion two.')
=>
foo
=>Expansion two.
define(`foo', `Second expansion two.')
=>
foo
=>Second expansion two.
undefine(`foo')
=>
foo
=>foo
Local variables within macros are made with 'pushdef' and 'popdef'.
At the start of the macro a new definition is pushed, within the macro
it is manipulated and at the end it is popped, revealing the former
definition.
It is possible to temporarily redefine a builtin with 'pushdef' and
'defn'.

File: m4.info, Node: Indir, Next: Builtin, Prev: Pushdef, Up: Definitions
5.7 Indirect call of macros
===========================
Any macro can be called indirectly with 'indir':
-- Builtin: indir (NAME, [ARGS...]
Results in a call to the macro NAME, which is passed the rest of
the arguments ARGS. If NAME is not defined, an error message is
printed, and the expansion is void.
The macro 'indir' is recognized only with parameters.
This can be used to call macros with computed or "invalid" names
('define' allows such names to be defined):
define(`$$internal$macro', `Internal macro (name `$0')')
=>
$$internal$macro
=>$$internal$macro
indir(`$$internal$macro')
=>Internal macro (name $$internal$macro)
The point is, here, that larger macro packages can have private
macros defined, that will not be called by accident. They can _only_ be
called through the builtin 'indir'.
One other point to observe is that argument collection occurs before
'indir' invokes NAME, so if argument collection changes the value of
NAME, that will be reflected in the final expansion. This is different
than the behavior when invoking macros directly, where the definition
that was in effect before argument collection is used.
$ m4 -d
define(`f', `1')
=>
f(define(`f', `2'))
=>1
indir(`f', define(`f', `3'))
=>3
indir(`f', undefine(`f'))
error->m4:stdin:4: undefined macro `f'
=>
When handed the result of 'defn' (*note Defn::) as one of its
arguments, 'indir' defers to the invoked NAME for whether a token
representing a builtin is recognized or flattened to the empty string.
$ m4 -d
indir(defn(`defn'), `divnum')
error->m4:stdin:1: Warning: indir: invalid macro name ignored
=>
indir(`define', defn(`defn'), `divnum')
error->m4:stdin:2: Warning: define: invalid macro name ignored
=>
indir(`define', `foo', defn(`divnum'))
=>
foo
=>0
indir(`divert', defn(`foo'))
error->m4:stdin:5: empty string treated as 0 in builtin `divert'
=>

File: m4.info, Node: Builtin, Prev: Indir, Up: Definitions
5.8 Indirect call of builtins
=============================
Builtin macros can be called indirectly with 'builtin':
-- Builtin: builtin (NAME, [ARGS...]
Results in a call to the builtin NAME, which is passed the rest of
the arguments ARGS. If NAME does not name a builtin, an error
message is printed, and the expansion is void.
The macro 'builtin' is recognized only with parameters.
This can be used even if NAME has been given another definition that
has covered the original, or been undefined so that no macro maps to the
builtin.
pushdef(`define', `hidden')
=>
undefine(`undefine')
=>
define(`foo', `bar')
=>hidden
foo
=>foo
builtin(`define', `foo', defn(`divnum'))
=>
foo
=>0
builtin(`define', `foo', `BAR')
=>
foo
=>BAR
undefine(`foo')
=>undefine(foo)
foo
=>BAR
builtin(`undefine', `foo')
=>
foo
=>foo
The NAME argument only matches the original name of the builtin, even
when the '--prefix-builtins' option (or '-P', *note Invoking m4:
Operation modes.) is in effect. This is different from 'indir', which
only tracks current macro names.
$ m4 -P
m4_builtin(`divnum')
=>0
m4_builtin(`m4_divnum')
error->m4:stdin:2: undefined builtin `m4_divnum'
=>
m4_indir(`divnum')
error->m4:stdin:3: undefined macro `divnum'
=>
m4_indir(`m4_divnum')
=>0
Note that 'indir' and 'builtin' can be used to invoke builtins
without arguments, even when they normally require parameters to be
recognized; but it will provoke a warning, and result in a void
expansion.
builtin
=>builtin
builtin()
error->m4:stdin:2: undefined builtin `'
=>
builtin(`builtin')
error->m4:stdin:3: Warning: too few arguments to builtin `builtin'
=>
builtin(`builtin',)
error->m4:stdin:4: undefined builtin `'
=>
builtin(`builtin', ``'
')
error->m4:stdin:5: undefined builtin ``'
error->'
=>
indir(`index')
error->m4:stdin:7: Warning: too few arguments to builtin `index'
=>

File: m4.info, Node: Conditionals, Next: Debugging, Prev: Definitions, Up: Top
6 Conditionals, loops, and recursion
************************************
Macros, expanding to plain text, perhaps with arguments, are not quite
enough. We would like to have macros expand to different things, based
on decisions taken at run-time. For that, we need some kind of
conditionals. Also, we would like to have some kind of loop construct,
so we could do something a number of times, or while some condition is
true.
* Menu:
* Ifdef:: Testing if a macro is defined
* Ifelse:: If-else construct, or multibranch
* Shift:: Recursion in 'm4'
* Forloop:: Iteration by counting
* Foreach:: Iteration by list contents
* Stacks:: Working with definition stacks
* Composition:: Building macros with macros

File: m4.info, Node: Ifdef, Next: Ifelse, Up: Conditionals
6.1 Testing if a macro is defined
=================================
There are two different builtin conditionals in 'm4'. The first is
'ifdef':
-- Builtin: ifdef (NAME, STRING-1, [STRING-2]
If NAME is defined as a macro, 'ifdef' expands to STRING-1,
otherwise to STRING-2. If STRING-2 is omitted, it is taken to be
the empty string (according to the normal rules).
The macro 'ifdef' is recognized only with parameters.
ifdef(`foo', ``foo' is defined', ``foo' is not defined')
=>foo is not defined
define(`foo', `')
=>
ifdef(`foo', ``foo' is defined', ``foo' is not defined')
=>foo is defined
ifdef(`no_such_macro', `yes', `no', `extra argument')
error->m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored
=>no

File: m4.info, Node: Ifelse, Next: Shift, Prev: Ifdef, Up: Conditionals
6.2 If-else construct, or multibranch
=====================================
The other conditional, 'ifelse', is much more powerful. It can be used
as a way to introduce a long comment, as an if-else construct, or as a
multibranch, depending on the number of arguments supplied:
-- Builtin: ifelse (COMMENT)
-- Builtin: ifelse (STRING-1, STRING-2, EQUAL, [NOT-EQUAL]
-- Builtin: ifelse (STRING-1, STRING-2, EQUAL-1, STRING-3, STRING-4,
EQUAL-2, ..., [NOT-EQUAL]
Used with only one argument, the 'ifelse' simply discards it and
produces no output.
If called with three or four arguments, 'ifelse' expands into
EQUAL, if STRING-1 and STRING-2 are equal (character for
character), otherwise it expands to NOT-EQUAL. A final fifth
argument is ignored, after triggering a warning.
If called with six or more arguments, and STRING-1 and STRING-2 are
equal, 'ifelse' expands into EQUAL-1, otherwise the first three
arguments are discarded and the processing starts again.
The macro 'ifelse' is recognized only with parameters.
Using only one argument is a common 'm4' idiom for introducing a
block comment, as an alternative to repeatedly using 'dnl'. This
special usage is recognized by GNU 'm4', so that in this case, the
warning about missing arguments is never triggered.
ifelse(`some comments')
=>
ifelse(`foo', `bar')
error->m4:stdin:2: Warning: too few arguments to builtin `ifelse'
=>
Using three or four arguments provides decision points.
ifelse(`foo', `bar', `true')
=>
ifelse(`foo', `foo', `true')
=>true
define(`foo', `bar')
=>
ifelse(foo, `bar', `true', `false')
=>true
ifelse(foo, `foo', `true', `false')
=>false
Notice how the first argument was used unquoted; it is common to
compare the expansion of a macro with a string. With this macro, you
can now reproduce the behavior of blind builtins, where the macro is
recognized only with arguments.
define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')')
=>
foo
=>foo
foo()
=>arguments:1
foo(`a', `b', `c')
=>arguments:3
For an example of a way to make defining blind macros easier, see
*note Composition::.
The macro 'ifelse' can take more than four arguments. If given more
than four arguments, 'ifelse' works like a 'case' or 'switch' statement
in traditional programming languages. If STRING-1 and STRING-2 are
equal, 'ifelse' expands into EQUAL-1, otherwise the procedure is
repeated with the first three arguments discarded. This calls for an
example:
ifelse(`foo', `bar', `third', `gnu', `gnats')
error->m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored
=>gnu
ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth')
=>
ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh')
=>seventh
ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8')
error->m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored
=>7
Naturally, the normal case will be slightly more advanced than these
examples. A common use of 'ifelse' is in macros implementing loops of
various kinds.

File: m4.info, Node: Shift, Next: Forloop, Prev: Ifelse, Up: Conditionals
6.3 Recursion in 'm4'
=====================
There is no direct support for loops in 'm4', but macros can be
recursive. There is no limit on the number of recursion levels, other
than those enforced by your hardware and operating system.
Loops can be programmed using recursion and the conditionals
described previously.
There is a builtin macro, 'shift', which can, among other things, be
used for iterating through the actual arguments to a macro:
-- Builtin: shift (ARG1, ...)
Takes any number of arguments, and expands to all its arguments
except ARG1, separated by commas, with each argument quoted.
The macro 'shift' is recognized only with parameters.
shift
=>shift
shift(`bar')
=>
shift(`foo', `bar', `baz')
=>bar,baz
An example of the use of 'shift' is this macro:
-- Composite: reverse (...)
Takes any number of arguments, and reverses their order.
It is implemented as:
define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'',
`reverse(shift($@)), `$1'')')
=>
reverse
=>
reverse(`foo')
=>foo
reverse(`foo', `bar', `gnats', `and gnus')
=>and gnus, gnats, bar, foo
While not a very interesting macro, it does show how simple loops can
be made with 'shift', 'ifelse' and recursion. It also shows that
'shift' is usually used with '$@'. Another example of this is an
implementation of a short-circuiting conditional operator.
-- Composite: cond (TEST-1, STRING-1, EQUAL-1, [TEST-2]
Similar to 'ifelse', where an equal comparison between the first
two strings results in the third, otherwise the first three
arguments are discarded and the process repeats. The difference is
that each TEST-<N> is expanded only when it is encountered. This
means that every third argument to 'cond' is normally given one
more level of quoting than the corresponding argument to 'ifelse'.
Here is the implementation of 'cond', along with a demonstration of
how it can short-circuit the side effects in 'side'. Notice how all the
unquoted side effects happen regardless of how many comparisons are made
with 'ifelse', compared with only the relevant effects with 'cond'.
define(`cond',
`ifelse(`$#', `1', `$1',
`ifelse($1, `$2', `$3',
`$0(shift(shift(shift($@))))')')')dnl
define(`side', `define(`counter', incr(counter))$1')dnl
define(`example1',
`define(`counter', `0')dnl
ifelse(side(`$1'), `yes', `one comparison: ',
side(`$1'), `no', `two comparisons: ',
side(`$1'), `maybe', `three comparisons: ',
`side(`default answer: ')')counter')dnl
define(`example2',
`define(`counter', `0')dnl
cond(`side(`$1')', `yes', `one comparison: ',
`side(`$1')', `no', `two comparisons: ',
`side(`$1')', `maybe', `three comparisons: ',
`side(`default answer: ')')counter')dnl
example1(`yes')
=>one comparison: 3
example1(`no')
=>two comparisons: 3
example1(`maybe')
=>three comparisons: 3
example1(`feeling rather indecisive today')
=>default answer: 4
example2(`yes')
=>one comparison: 1
example2(`no')
=>two comparisons: 2
example2(`maybe')
=>three comparisons: 3
example2(`feeling rather indecisive today')
=>default answer: 4
Another common task that requires iteration is joining a list of
arguments into a single string.
-- Composite: join ([SEPARATOR]
-- Composite: joinall ([SEPARATOR]
Generate a single-quoted string, consisting of each ARG separated
by SEPARATOR. While 'joinall' always outputs a SEPARATOR between
arguments, 'join' avoids the SEPARATOR for an empty ARG.
Here are some examples of its usage, based on the implementation
'm4-1.4.17/examples/join.m4' distributed in this package:
$ m4 -I examples
include(`join.m4')
=>
join,join(`-'),join(`-', `'),join(`-', `', `')
=>,,,
joinall,joinall(`-'),joinall(`-', `'),joinall(`-', `', `')
=>,,,-
join(`-', `1')
=>1
join(`-', `1', `2', `3')
=>1-2-3
join(`', `1', `2', `3')
=>123
join(`-', `', `1', `', `', `2', `')
=>1-2
joinall(`-', `', `1', `', `', `2', `')
=>-1---2-
join(`,', `1', `2', `3')
=>1,2,3
define(`nargs', `$#')dnl
nargs(join(`,', `1', `2', `3'))
=>1
Examining the implementation shows some interesting points about
several m4 programming idioms.
$ m4 -I examples
undivert(`join.m4')dnl
=>divert(`-1')
=># join(sep, args) - join each non-empty ARG into a single
=># string, with each element separated by SEP
=>define(`join',
=>`ifelse(`$#', `2', ``$2'',
=> `ifelse(`$2', `', `', ``$2'_')$0(`$1', shift(shift($@)))')')
=>define(`_join',
=>`ifelse(`$#$2', `2', `',
=> `ifelse(`$2', `', `', ``$1$2'')$0(`$1', shift(shift($@)))')')
=># joinall(sep, args) - join each ARG, including empty ones,
=># into a single string, with each element separated by SEP
=>define(`joinall', ``$2'_$0(`$1', shift($@))')
=>define(`_joinall',
=>`ifelse(`$#', `2', `', ``$1$3'$0(`$1', shift(shift($@)))')')
=>divert`'dnl
First, notice that this implementation creates helper macros '_join'
and '_joinall'. This division of labor makes it easier to output the
correct number of SEPARATOR instances: 'join' and 'joinall' are
responsible for the first argument, without a separator, while '_join'
and '_joinall' are responsible for all remaining arguments, always
outputting a separator when outputting an argument.
Next, observe how 'join' decides to iterate to itself, because the
first ARG was empty, or to output the argument and swap over to '_join'.
If the argument is non-empty, then the nested 'ifelse' results in an
unquoted '_', which is concatenated with the '$0' to form the next macro
name to invoke. The 'joinall' implementation is simpler since it does
not have to suppress empty ARG; it always executes once then defers to
'_joinall'.
Another important idiom is the idea that SEPARATOR is reused for each
iteration. Each iteration has one less argument, but rather than
discarding '$1' by iterating with '$0(shift($@))', the macro discards
'$2' by using '$0(`$1', shift(shift($@)))'.
Next, notice that it is possible to compare more than one condition
in a single 'ifelse' test. The test of '$#$2' against '2' allows
'_join' to iterate for two separate reasons--either there are still more
than two arguments, or there are exactly two arguments but the last
argument is not empty.
Finally, notice that these macros require exactly two arguments to
terminate recursion, but that they still correctly result in empty
output when given no ARGS (i.e., zero or one macro argument). On the
first pass when there are too few arguments, the 'shift' results in no
output, but leaves an empty string to serve as the required second
argument for the second pass. Put another way, '`$1', shift($@)' is not
the same as '$@', since only the former guarantees at least two
arguments.
Sometimes, a recursive algorithm requires adding quotes to each
element, or treating multiple arguments as a single element:
-- Composite: quote (...)
-- Composite: dquote (...)
-- Composite: dquote_elt (...)
Takes any number of arguments, and adds quoting. With 'quote',
only one level of quoting is added, effectively removing whitespace
after commas and turning multiple arguments into a single string.
With 'dquote', two levels of quoting are added, one around each
element, and one around the list. And with 'dquote_elt', two
levels of quoting are added around each element.
An actual implementation of these three macros is distributed as
'm4-1.4.17/examples/quote.m4' in this package. First, let's examine
their usage:
$ m4 -I examples
include(`quote.m4')
=>
-quote-dquote-dquote_elt-
=>----
-quote()-dquote()-dquote_elt()-
=>--`'-`'-
-quote(`1')-dquote(`1')-dquote_elt(`1')-
=>-1-`1'-`1'-
-quote(`1', `2')-dquote(`1', `2')-dquote_elt(`1', `2')-
=>-1,2-`1',`2'-`1',`2'-
define(`n', `$#')dnl
-n(quote(`1', `2'))-n(dquote(`1', `2'))-n(dquote_elt(`1', `2'))-
=>-1-1-2-
dquote(dquote_elt(`1', `2'))
=>``1'',``2''
dquote_elt(dquote(`1', `2'))
=>``1',`2''
The last two lines show that when given two arguments, 'dquote'
results in one string, while 'dquote_elt' results in two. Now, examine
the implementation. Note that 'quote' and 'dquote_elt' make decisions
based on their number of arguments, so that when called without
arguments, they result in nothing instead of a quoted empty string; this
is so that it is possible to distinguish between no arguments and an
empty first argument. 'dquote', on the other hand, results in a string
no matter what, since it is still possible to tell whether it was
invoked without arguments based on the resulting string.
$ m4 -I examples
undivert(`quote.m4')dnl
=>divert(`-1')
=># quote(args) - convert args to single-quoted string
=>define(`quote', `ifelse(`$#', `0', `', ``$*'')')
=># dquote(args) - convert args to quoted list of quoted strings
=>define(`dquote', ``$@'')
=># dquote_elt(args) - convert args to list of double-quoted strings
=>define(`dquote_elt', `ifelse(`$#', `0', `', `$#', `1', ```$1''',
=> ```$1'',$0(shift($@))')')
=>divert`'dnl
It is worth pointing out that 'quote(ARGS)' is more efficient than
'joinall(`,', ARGS)' for producing the same output.
One more useful macro based on 'shift' allows portably selecting an
arbitrary argument (usually greater than the ninth argument), without
relying on the GNU extension of multi-digit arguments (*note
Arguments::).
-- Composite: argn (N, ...)
Expands to argument N out of the remaining arguments. N must be a
positive number. Usually invoked as 'argn(`N',$@)'.
It is implemented as:
define(`argn', `ifelse(`$1', 1, ``$2'',
`argn(decr(`$1'), shift(shift($@)))')')
=>
argn(`1', `a')
=>a
define(`foo', `argn(`11', $@)')
=>
foo(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k', `l')
=>k

File: m4.info, Node: Forloop, Next: Foreach, Prev: Shift, Up: Conditionals
6.4 Iteration by counting
=========================
Here is an example of a loop macro that implements a simple for loop.
-- Composite: forloop (ITERATOR, START, END, TEXT)
Takes the name in ITERATOR, which must be a valid macro name, and
successively assign it each integer value from START to END,
inclusive. For each assignment to ITERATOR, append TEXT to the
expansion of the 'forloop'. TEXT may refer to ITERATOR. Any
definition of ITERATOR prior to this invocation is restored.
It can, for example, be used for simple counting:
$ m4 -I examples
include(`forloop.m4')
=>
forloop(`i', `1', `8', `i ')
=>1 2 3 4 5 6 7 8
For-loops can be nested, like:
$ m4 -I examples
include(`forloop.m4')
=>
forloop(`i', `1', `4', `forloop(`j', `1', `8', ` (i, j)')
')
=> (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) (1, 7) (1, 8)
=> (2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (2, 7) (2, 8)
=> (3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6) (3, 7) (3, 8)
=> (4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6) (4, 7) (4, 8)
=>
The implementation of the 'forloop' macro is fairly straightforward.
The 'forloop' macro itself is simply a wrapper, which saves the previous
definition of the first argument, calls the internal macro '_forloop',
and re-establishes the saved definition of the first argument.
The macro '_forloop' expands the fourth argument once, and tests to
see if the iterator has reached the final value. If it has not
finished, it increments the iterator (using the predefined macro 'incr',
*note Incr::), and recurses.
Here is an actual implementation of 'forloop', distributed as
'm4-1.4.17/examples/forloop.m4' in this package:
$ m4 -I examples
undivert(`forloop.m4')dnl
=>divert(`-1')
=># forloop(var, from, to, stmt) - simple version
=>define(`forloop', `pushdef(`$1', `$2')_forloop($@)popdef(`$1')')
=>define(`_forloop',
=> `$4`'ifelse($1, `$3', `', `define(`$1', incr($1))$0($@)')')
=>divert`'dnl
Notice the careful use of quotes. Certain macro arguments are left
unquoted, each for its own reason. Try to find out _why_ these
arguments are left unquoted, and see what happens if they are quoted.
(As presented, these two macros are useful but not very robust for
general use. They lack even basic error handling for cases like START
less than END, END not numeric, or ITERATOR not being a macro name. See
if you can improve these macros; or *note Answers: Improved forloop.).

File: m4.info, Node: Foreach, Next: Stacks, Prev: Forloop, Up: Conditionals
6.5 Iteration by list contents
==============================
Here is an example of a loop macro that implements list iteration.
-- Composite: foreach (ITERATOR, PAREN-LIST, TEXT)
-- Composite: foreachq (ITERATOR, QUOTE-LIST, TEXT)
Takes the name in ITERATOR, which must be a valid macro name, and
successively assign it each value from PAREN-LIST or QUOTE-LIST.
In 'foreach', PAREN-LIST is a comma-separated list of elements
contained in parentheses. In 'foreachq', QUOTE-LIST is a
comma-separated list of elements contained in a quoted string. For
each assignment to ITERATOR, append TEXT to the overall expansion.
TEXT may refer to ITERATOR. Any definition of ITERATOR prior to
this invocation is restored.
As an example, this displays each word in a list inside of a
sentence, using an implementation of 'foreach' distributed as
'm4-1.4.17/examples/foreach.m4', and 'foreachq' in
'm4-1.4.17/examples/foreachq.m4'.
$ m4 -I examples
include(`foreach.m4')
=>
foreach(`x', (foo, bar, foobar), `Word was: x
')dnl
=>Word was: foo
=>Word was: bar
=>Word was: foobar
include(`foreachq.m4')
=>
foreachq(`x', `foo, bar, foobar', `Word was: x
')dnl
=>Word was: foo
=>Word was: bar
=>Word was: foobar
It is possible to be more complex; each element of the PAREN-LIST or
QUOTE-LIST can itself be a list, to pass as further arguments to a
helper macro. This example generates a shell case statement:
$ m4 -I examples
include(`foreach.m4')
=>
define(`_case', ` $1)
$2=" $1";;
')dnl
define(`_cat', `$1$2')dnl
case $`'1 in
=>case $1 in
foreach(`x', `(`(`a', `vara')', `(`b', `varb')', `(`c', `varc')')',
`_cat(`_case', x)')dnl
=> a)
=> vara=" a";;
=> b)
=> varb=" b";;
=> c)
=> varc=" c";;
esac
=>esac
The implementation of the 'foreach' macro is a bit more involved; it
is a wrapper around two helper macros. First, '_arg1' is needed to grab
the first element of a list. Second, '_foreach' implements the
recursion, successively walking through the original list. Here is a
simple implementation of 'foreach':
$ m4 -I examples
undivert(`foreach.m4')dnl
=>divert(`-1')
=># foreach(x, (item_1, item_2, ..., item_n), stmt)
=># parenthesized list, simple version
=>define(`foreach', `pushdef(`$1')_foreach($@)popdef(`$1')')
=>define(`_arg1', `$1')
=>define(`_foreach', `ifelse(`$2', `()', `',
=> `define(`$1', _arg1$2)$3`'$0(`$1', (shift$2), `$3')')')
=>divert`'dnl
Unfortunately, that implementation is not robust to macro names as
list elements. Each iteration of '_foreach' is stripping another layer
of quotes, leading to erratic results if list elements are not already
fully expanded. The first cut at implementing 'foreachq' takes this
into account. Also, when using quoted elements in a PAREN-LIST, the
overall list must be quoted. A QUOTE-LIST has the nice property of
requiring fewer characters to create a list containing the same quoted
elements. To see the difference between the two macros, we attempt to
pass double-quoted macro names in a list, expecting the macro name on
output after one layer of quotes is removed during list iteration and
the final layer removed during the final rescan:
$ m4 -I examples
define(`a', `1')define(`b', `2')define(`c', `3')
=>
include(`foreach.m4')
=>
include(`foreachq.m4')
=>
foreach(`x', `(``a'', ``(b'', ``c)'')', `x
')
=>1
=>(2)1
=>
=>, x
=>)
foreachq(`x', ```a'', ``(b'', ``c)''', `x
')dnl
=>a
=>(b
=>c)
Obviously, 'foreachq' did a better job; here is its implementation:
$ m4 -I examples
undivert(`foreachq.m4')dnl
=>include(`quote.m4')dnl
=>divert(`-1')
=># foreachq(x, `item_1, item_2, ..., item_n', stmt)
=># quoted list, simple version
=>define(`foreachq', `pushdef(`$1')_foreachq($@)popdef(`$1')')
=>define(`_arg1', `$1')
=>define(`_foreachq', `ifelse(quote($2), `', `',
=> `define(`$1', `_arg1($2)')$3`'$0(`$1', `shift($2)', `$3')')')
=>divert`'dnl
Notice that '_foreachq' had to use the helper macro 'quote' defined
earlier (*note Shift::), to ensure that the embedded 'ifelse' call does
not go haywire if a list element contains a comma. Unfortunately, this
implementation of 'foreachq' has its own severe flaw. Whereas the
'foreach' implementation was linear, this macro is quadratic in the
number of list elements, and is much more likely to trip up the limit
set by the command line option '--nesting-limit' (or '-L', *note
Invoking m4: Limits control.). Additionally, this implementation does
not expand 'defn(`ITERATOR')' very well, when compared with 'foreach'.
$ m4 -I examples
include(`foreach.m4')include(`foreachq.m4')
=>
foreach(`name', `(`a', `b')', ` defn(`name')')
=> a b
foreachq(`name', ``a', `b'', ` defn(`name')')
=> _arg1(`a', `b') _arg1(shift(`a', `b'))
It is possible to have robust iteration with linear behavior and sane
ITERATOR contents for either list style. See if you can learn from the
best elements of both of these implementations to create robust macros
(or *note Answers: Improved foreach.).

File: m4.info, Node: Stacks, Next: Composition, Prev: Foreach, Up: Conditionals
6.6 Working with definition stacks
==================================
Thanks to 'pushdef', manipulation of a stack is an intrinsic operation
in 'm4'. Normally, only the topmost definition in a stack is important,
but sometimes, it is desirable to manipulate the entire definition
stack.
-- Composite: stack_foreach (MACRO, ACTION)
-- Composite: stack_foreach_lifo (MACRO, ACTION)
For each of the 'pushdef' definitions associated with MACRO, invoke
the macro ACTION with a single argument of that definition.
'stack_foreach' visits the oldest definition first, while
'stack_foreach_lifo' visits the current definition first. ACTION
should not modify or dereference MACRO. There are a few special
macros, such as 'defn', which cannot be used as the MACRO
parameter.
A sample implementation of these macros is distributed in the file
'm4-1.4.17/examples/stack.m4'.
$ m4 -I examples
include(`stack.m4')
=>
pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3')
=>
define(`show', ``$1'
')
=>
stack_foreach(`a', `show')dnl
=>1
=>2
=>3
stack_foreach_lifo(`a', `show')dnl
=>3
=>2
=>1
Now for the implementation. Note the definition of a helper macro,
'_stack_reverse', which destructively swaps the contents of one stack of
definitions into the reverse order in the temporary macro 'tmp-$1'. By
calling the helper twice, the original order is restored back into the
macro '$1'; since the operation is destructive, this explains why '$1'
must not be modified or dereferenced during the traversal. The caller
can then inject additional code to pass the definition currently being
visited to '$2'. The choice of helper names is intentional; since '-'
is not valid as part of a macro name, there is no risk of conflict with
a valid macro name, and the code is guaranteed to use 'defn' where
necessary. Finally, note that any macro used in the traversal of a
'pushdef' stack, such as 'pushdef' or 'defn', cannot be handled by
'stack_foreach', since the macro would temporarily be undefined during
the algorithm.
$ m4 -I examples
undivert(`stack.m4')dnl
=>divert(`-1')
=># stack_foreach(macro, action)
=># Invoke ACTION with a single argument of each definition
=># from the definition stack of MACRO, starting with the oldest.
=>define(`stack_foreach',
=>`_stack_reverse(`$1', `tmp-$1')'dnl
=>`_stack_reverse(`tmp-$1', `$1', `$2(defn(`$1'))')')
=># stack_foreach_lifo(macro, action)
=># Invoke ACTION with a single argument of each definition
=># from the definition stack of MACRO, starting with the newest.
=>define(`stack_foreach_lifo',
=>`_stack_reverse(`$1', `tmp-$1', `$2(defn(`$1'))')'dnl
=>`_stack_reverse(`tmp-$1', `$1')')
=>define(`_stack_reverse',
=>`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0($@)')')
=>divert`'dnl

File: m4.info, Node: Composition, Prev: Stacks, Up: Conditionals
6.7 Building macros with macros
===============================
Since m4 is a macro language, it is possible to write macros that can
build other macros. First on the list is a way to automate the creation
of blind macros.
-- Composite: define_blind (NAME, [VALUE]
Defines NAME as a blind macro, such that NAME will expand to VALUE
only when given explicit arguments. VALUE should not be the result
of 'defn' (*note Defn::). This macro is only recognized with
parameters, and results in an empty string.
Defining a macro to define another macro can be a bit tricky. We
want to use a literal '$#' in the argument to the nested 'define'.
However, if '$' and '#' are adjacent in the definition of
'define_blind', then it would be expanded as the number of arguments to
'define_blind' rather than the intended number of arguments to NAME.
The solution is to pass the difficult characters through extra arguments
to a helper macro '_define_blind'. When composing macros, it is a
common idiom to need a helper macro to concatenate text that forms
parameters in the composed macro, rather than interpreting the text as a
parameter of the composing macro.
As for the limitation against using 'defn', there are two reasons.
If a macro was previously defined with 'define_blind', then it can
safely be renamed to a new blind macro using plain 'define'; using
'define_blind' to rename it just adds another layer of 'ifelse',
occupying memory and slowing down execution. And if a macro is a
builtin, then it would result in an attempt to define a macro consisting
of both text and a builtin token; this is not supported, and the builtin
token is flattened to an empty string.
With that explanation, here's the definition, and some sample usage.
Notice that 'define_blind' is itself a blind macro.
$ m4 -d
define(`define_blind', `ifelse(`$#', `0', ``$0'',
`_$0(`$1', `$2', `$'`#', `$'`0')')')
=>
define(`_define_blind', `define(`$1',
`ifelse(`$3', `0', ``$4'', `$2')')')
=>
define_blind
=>define_blind
define_blind(`foo', `arguments were $*')
=>
foo
=>foo
foo(`bar')
=>arguments were bar
define(`blah', defn(`foo'))
=>
blah
=>blah
blah(`a', `b')
=>arguments were a,b
defn(`blah')
=>ifelse(`$#', `0', ``$0'', `arguments were $*')
Another interesting composition tactic is argument "currying", or
factoring a macro that takes multiple arguments for use in a context
that provides exactly one argument.
-- Composite: curry (MACRO, ...)
Expand to a macro call that takes exactly one argument, then
appends that argument to the original arguments and invokes MACRO
with the resulting list of arguments.
A demonstration of currying makes the intent of this macro a little
more obvious. The macro 'stack_foreach' mentioned earlier is an example
of a context that provides exactly one argument to a macro name. But
coupled with currying, we can invoke 'reverse' with two arguments for
each definition of a macro stack. This example uses the file
'm4-1.4.17/examples/curry.m4' included in the distribution.
$ m4 -I examples
include(`curry.m4')include(`stack.m4')
=>
define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'',
`reverse(shift($@)), `$1'')')
=>
pushdef(`a', `1')pushdef(`a', `2')pushdef(`a', `3')
=>
stack_foreach(`a', `:curry(`reverse', `4')')
=>:1, 4:2, 4:3, 4
curry(`curry', `reverse', `1')(`2')(`3')
=>3, 2, 1
Now for the implementation. Notice how 'curry' leaves off with a
macro name but no open parenthesis, while still in the middle of
collecting arguments for '$1'. The macro '_curry' is the helper macro
that takes one argument, then adds it to the list and finally supplies
the closing parenthesis. The use of a comma inside the 'shift' call
allows currying to also work for a macro that takes one argument,
although it often makes more sense to invoke that macro directly rather
than going through 'curry'.
$ m4 -I examples
undivert(`curry.m4')dnl
=>divert(`-1')
=># curry(macro, args)
=># Expand to a macro call that takes one argument, then invoke
=># macro(args, extra).
=>define(`curry', `$1(shift($@,)_$0')
=>define(`_curry', ``$1')')
=>divert`'dnl
Unfortunately, with M4 1.4.x, 'curry' is unable to handle builtin
tokens, which are silently flattened to the empty string when passed
through another text macro. This limitation will be lifted in a future
release of M4.
Putting the last few concepts together, it is possible to copy or
rename an entire stack of macro definitions.
-- Composite: copy (SOURCE, DEST)
-- Composite: rename (SOURCE, DEST)
Ensure that DEST is undefined, then define it to the same stack of
definitions currently in SOURCE. 'copy' leaves SOURCE unchanged,
while 'rename' undefines SOURCE. There are only a few macros, such
as 'copy' or 'defn', which cannot be copied via this macro.
The implementation is relatively straightforward (although since it
uses 'curry', it is unable to copy builtin macros, such as the second
definition of 'a' as a synonym for 'divnum'. See if you can design a
version that works around this limitation, or *note Answers: Improved
copy.).
$ m4 -I examples
include(`curry.m4')include(`stack.m4')
=>
define(`rename', `copy($@)undefine(`$1')')dnl
define(`copy', `ifdef(`$2', `errprint(`$2 already defined
')m4exit(`1')',
`stack_foreach(`$1', `curry(`pushdef', `$2')')')')dnl
pushdef(`a', `1')pushdef(`a', defn(`divnum'))pushdef(`a', `2')
=>
copy(`a', `b')
=>
rename(`b', `c')
=>
a b c
=>2 b 2
popdef(`a', `c')c a
=> 0
popdef(`a', `c')a c
=>1 1

File: m4.info, Node: Debugging, Next: Input Control, Prev: Conditionals, Up: Top
7 How to debug macros and input
*******************************
When writing macros for 'm4', they often do not work as intended on the
first try (as is the case with most programming languages).
Fortunately, there is support for macro debugging in 'm4'.
* Menu:
* Dumpdef:: Displaying macro definitions
* Trace:: Tracing macro calls
* Debug Levels:: Controlling debugging output
* Debug Output:: Saving debugging output

File: m4.info, Node: Dumpdef, Next: Trace, Up: Debugging
7.1 Displaying macro definitions
================================
If you want to see what a name expands into, you can use the builtin
'dumpdef':
-- Builtin: dumpdef ([NAMES...]
Accepts any number of arguments. If called without any arguments,
it displays the definitions of all known names, otherwise it
displays the definitions of the NAMES given. The output is printed
to the current debug file (usually standard error), and is sorted
by name. If an unknown name is encountered, a warning is printed.
The expansion of 'dumpdef' is void.
$ m4 -d
define(`foo', `Hello world.')
=>
dumpdef(`foo')
error->foo: =>
dumpdef(`define')
error->define: =>
The last example shows how builtin macros definitions are displayed.
The definition that is dumped corresponds to what would occur if the
macro were to be called at that point, even if other definitions are
still live due to redefining a macro during argument collection.
$ m4 -d
pushdef(`f', ``$0'1')pushdef(`f', ``$0'2')
=>
f(popdef(`f')dumpdef(`f'))
error->f: =>f2
f(popdef(`f')dumpdef(`f'))
error->m4:stdin:3: undefined macro `f'
=>f1
*Note Debug Levels::, for information on controlling the details of
the display.

File: m4.info, Node: Trace, Next: Debug Levels, Prev: Dumpdef, Up: Debugging
7.2 Tracing macro calls
=======================
It is possible to trace macro calls and expansions through the builtins
'traceon' and 'traceoff':
-- Builtin: traceon ([NAMES...]
-- Builtin: traceoff ([NAMES...]
When called without any arguments, 'traceon' and 'traceoff' will
turn tracing on and off, respectively, for all currently defined
macros.
When called with arguments, only the macros listed in NAMES are
affected, whether or not they are currently defined.
The expansion of 'traceon' and 'traceoff' is void.
Whenever a traced macro is called and the arguments have been
collected, the call is displayed. If the expansion of the macro call is
not void, the expansion can be displayed after the call. The output is
printed to the current debug file (defaulting to standard error, *note
Debug Output::).
$ m4 -d
define(`foo', `Hello World.')
=>
define(`echo', `$@')
=>
traceon(`foo', `echo')
=>
foo
error->m4trace: -1- foo -> `Hello World.'
=>Hello World.
echo(`gnus', `and gnats')
error->m4trace: -1- echo(`gnus', `and gnats') -> ``gnus',`and gnats''
=>gnus,and gnats
The number between dashes is the depth of the expansion. It is one
most of the time, signifying an expansion at the outermost level, but it
increases when macro arguments contain unquoted macro calls. The
maximum number that will appear between dashes is controlled by the
option '--nesting-limit' (or '-L', *note Invoking m4: Limits control.).
Additionally, the option '--trace' (or '-t') can be used to invoke
'traceon(NAME)' before parsing input.
$ m4 -L 3 -t ifelse
ifelse(`one level')
error->m4trace: -1- ifelse
=>
ifelse(ifelse(ifelse(`three levels')))
error->m4trace: -3- ifelse
error->m4trace: -2- ifelse
error->m4trace: -1- ifelse
=>
ifelse(ifelse(ifelse(ifelse(`four levels'))))
error->m4:stdin:3: recursion limit of 3 exceeded, use -L<N> to change it
Tracing by name is an attribute that is preserved whether the macro
is defined or not. This allows the selection of macros to trace before
those macros are defined.
$ m4 -d
traceoff(`foo')
=>
traceon(`foo')
=>
foo
=>foo
defn(`foo')
=>
define(`foo', `bar')
=>
foo
error->m4trace: -1- foo -> `bar'
=>bar
undefine(`foo')
=>
ifdef(`foo', `yes', `no')
=>no
indir(`foo')
error->m4:stdin:9: undefined macro `foo'
=>
define(`foo', `blah')
=>
foo
error->m4trace: -1- foo -> `blah'
=>blah
traceoff
=>
foo
=>blah
Tracing even works on builtins. However, 'defn' (*note Defn::) does
not transfer tracing status.
$ m4 -d
traceon(`traceon')
=>
traceon(`traceoff')
error->m4trace: -1- traceon(`traceoff')
=>
traceoff(`traceoff')
error->m4trace: -1- traceoff(`traceoff')
=>
traceoff(`traceon')
=>
traceon(`eval', `m4_divnum')
=>
define(`m4_eval', defn(`eval'))
=>
define(`m4_divnum', defn(`divnum'))
=>
eval(divnum)
error->m4trace: -1- eval(`0') -> `0'
=>0
m4_eval(m4_divnum)
error->m4trace: -2- m4_divnum -> `0'
=>0
*Note Debug Levels::, for information on controlling the details of
the display. The format of the trace output is not specified by POSIX,
and varies between implementations of 'm4'.

File: m4.info, Node: Debug Levels, Next: Debug Output, Prev: Trace, Up: Debugging
7.3 Controlling debugging output
================================
The '-d' option to 'm4' (or '--debug', *note Invoking m4: Debugging
options.) controls the amount of details presented in three categories
of output. Trace output is requested by 'traceon' (*note Trace::), and
each line is prefixed by 'm4trace:' in relation to a macro invocation.
Debug output tracks useful events not associated with a macro
invocation, and each line is prefixed by 'm4debug:'. Finally, 'dumpdef'
(*note Dumpdef::) output is affected, with no prefix added to the output
lines.
The FLAGS following the option can be one or more of the following:
'a'
In trace output, show the actual arguments that were collected
before invoking the macro. This applies to all macro calls if the
't' flag is used, otherwise only the macros covered by calls of
'traceon'. Arguments are subject to length truncation specified by
the command line option '--arglength' (or '-l').
'c'
In trace output, show several trace lines for each macro call. A
line is shown when the macro is seen, but before the arguments are
collected; a second line when the arguments have been collected and
a third line after the call has completed.
'e'
In trace output, show the expansion of each macro call, if it is
not void. This applies to all macro calls if the 't' flag is used,
otherwise only the macros covered by calls of 'traceon'. The
expansion is subject to length truncation specified by the command
line option '--arglength' (or '-l').
'f'
In debug and trace output, include the name of the current input
file in the output line.
'i'
In debug output, print a message each time the current input file
is changed.
'l'
In debug and trace output, include the current input line number in
the output line.
'p'
In debug output, print a message when a named file is found through
the path search mechanism (*note Search Path::), giving the actual
file name used.
'q'
In trace and dumpdef output, quote actual arguments and macro
expansions in the display with the current quotes. This is useful
in connection with the 'a' and 'e' flags above.
't'
In trace output, trace all macro calls made in this invocation of
'm4', regardless of the settings of 'traceon'.
'x'
In trace output, add a unique 'macro call id' to each line of the
trace output. This is useful in connection with the 'c' flag
above.
'V'
A shorthand for all of the above flags.
If no flags are specified with the '-d' option, the default is 'aeq'.
The examples throughout this manual assume the default flags.
There is a builtin macro 'debugmode', which allows on-the-fly control
of the debugging output format:
-- Builtin: debugmode ([FLAGS]
The argument FLAGS should be a subset of the letters listed above.
As special cases, if the argument starts with a '+', the flags are
added to the current debug flags, and if it starts with a '-', they
are removed. If no argument is present, all debugging flags are
cleared (as if no '-d' was given), and with an empty argument the
flags are reset to the default of 'aeq'.
The expansion of 'debugmode' is void.
$ m4
define(`foo', `FOO')
=>
traceon(`foo')
=>
debugmode()
=>
foo
error->m4trace: -1- foo -> `FOO'
=>FOO
debugmode
=>
foo
error->m4trace: -1- foo
=>FOO
debugmode(`+l')
=>
foo
error->m4trace:8: -1- foo
=>FOO
The following example demonstrates the behavior of length truncation,
when specified on the command line. Note that each argument and the
final result are individually truncated. Also, the special tokens for
builtin functions are not truncated.
$ m4 -d -l 6
define(`echo', `$@')debugmode(`+t')
=>
echo(`1', `long string')
error->m4trace: -1- echo(`1', `long s...') -> ``1',`l...'
=>1,long string
indir(`echo', defn(`changequote'))
error->m4trace: -2- defn(`change...')
error->m4trace: -1- indir(`echo', <changequote>) -> ``''
=>
This example shows the effects of the debug flags that are not
related to macro tracing.
$ m4 -dip -I examples
error->m4debug: input read from stdin
include(`foo')dnl
error->m4debug: path search for `foo' found `examples/foo'
error->m4debug: input read from examples/foo
=>bar
error->m4debug: input reverted to stdin, line 1
^D
error->m4debug: input exhausted

File: m4.info, Node: Debug Output, Prev: Debug Levels, Up: Debugging
7.4 Saving debugging output
===========================
Debug and tracing output can be redirected to files using either the
'--debugfile' option to 'm4' (*note Invoking m4: Debugging options.), or
with the builtin macro 'debugfile':
-- Builtin: debugfile ([FILE]
Sends all further debug and trace output to FILE, opened in append
mode. If FILE is the empty string, debug and trace output are
discarded. If 'debugfile' is called without any arguments, debug
and trace output are sent to standard error. This does not affect
warnings, error messages, or 'errprint' output, which are always
sent to standard error. If FILE cannot be opened, the current
debug file is unchanged, and an error is issued.
The expansion of 'debugfile' is void.
$ m4 -d
traceon(`divnum')
=>
divnum(`extra')
error->m4:stdin:2: Warning: excess arguments to builtin `divnum' ignored
error->m4trace: -1- divnum(`extra') -> `0'
=>0
debugfile()
=>
divnum(`extra')
error->m4:stdin:4: Warning: excess arguments to builtin `divnum' ignored
=>0
debugfile
=>
divnum
error->m4trace: -1- divnum -> `0'
=>0

File: m4.info, Node: Input Control, Next: File Inclusion, Prev: Debugging, Up: Top
8 Input control
***************
This chapter describes various builtin macros for controlling the input
to 'm4'.
* Menu:
* Dnl:: Deleting whitespace in input
* Changequote:: Changing the quote characters
* Changecom:: Changing the comment delimiters
* Changeword:: Changing the lexical structure of words
* M4wrap:: Saving text until end of input

File: m4.info, Node: Dnl, Next: Changequote, Up: Input Control
8.1 Deleting whitespace in input
================================
The builtin 'dnl' stands for "Discard to Next Line":
-- Builtin: dnl
All characters, up to and including the next newline, are discarded
without performing any macro expansion. A warning is issued if the
end of the file is encountered without a newline.
The expansion of 'dnl' is void.
It is often used in connection with 'define', to remove the newline
that follows the call to 'define'. Thus
define(`foo', `Macro `foo'.')dnl A very simple macro, indeed.
foo
=>Macro foo.
The input up to and including the next newline is discarded, as
opposed to the way comments are treated (*note Comments::).
Usually, 'dnl' is immediately followed by an end of line or some
other whitespace. GNU 'm4' will produce a warning diagnostic if 'dnl'
is followed by an open parenthesis. In this case, 'dnl' will collect
and process all arguments, looking for a matching close parenthesis.
All predictable side effects resulting from this collection will take
place. 'dnl' will return no output. The input following the matching
close parenthesis up to and including the next newline, on whatever line
containing it, will still be discarded.
dnl(`args are ignored, but side effects occur',
define(`foo', `like this')) while this text is ignored: undefine(`foo')
error->m4:stdin:1: Warning: excess arguments to builtin `dnl' ignored
See how `foo' was defined, foo?
=>See how foo was defined, like this?
If the end of file is encountered without a newline character, a
warning is issued and dnl stops consuming input.
m4wrap(`m4wrap(`2 hi
')0 hi dnl 1 hi')
=>
define(`hi', `HI')
=>
^D
error->m4:stdin:1: Warning: end of file treated as newline
=>0 HI 2 HI

File: m4.info, Node: Changequote, Next: Changecom, Prev: Dnl, Up: Input Control
8.2 Changing the quote characters
=================================
The default quote delimiters can be changed with the builtin
'changequote':
-- Builtin: changequote ([START = '`']
This sets START as the new begin-quote delimiter and END as the new
end-quote delimiter. If both arguments are missing, the default
quotes ('`' and ''') are used. If START is void, then quoting is
disabled. Otherwise, if END is missing or void, the default
end-quote delimiter (''') is used. The quote delimiters can be of
any length.
The expansion of 'changequote' is void.
changequote(`[', `]')
=>
define([foo], [Macro [foo].])
=>
foo
=>Macro foo.
The quotation strings can safely contain eight-bit characters. If no
single character is appropriate, START and END can be of any length.
Other implementations cap the delimiter length to five characters, but
GNU has no inherent limit.
changequote(`[[[', `]]]')
=>
define([[[foo]]], [[[Macro [[[[[foo]]]]].]]])
=>
foo
=>Macro [[foo]].
Calling 'changequote' with START as the empty string will effectively
disable the quoting mechanism, leaving no way to quote text. However,
using an empty string is not portable, as some other implementations of
'm4' revert to the default quoting, while others preserve the prior
non-empty delimiter. If START is not empty, then an empty END will use
the default end-quote delimiter of ''', as otherwise, it would be
impossible to end a quoted string. Again, this is not portable, as some
other 'm4' implementations reuse START as the end-quote delimiter, while
others preserve the previous non-empty value. Omitting both arguments
restores the default begin-quote and end-quote delimiters; fortunately
this behavior is portable to all implementations of 'm4'.
define(`foo', `Macro `FOO'.')
=>
changequote(`', `')
=>
foo
=>Macro `FOO'.
`foo'
=>`Macro `FOO'.'
changequote(`,)
=>
foo
=>Macro FOO.
There is no way in 'm4' to quote a string containing an unmatched
begin-quote, except using 'changequote' to change the current quotes.
If the quotes should be changed from, say, '[' to '[[', temporary
quote characters have to be defined. To achieve this, two calls of
'changequote' must be made, one for the temporary quotes and one for the
new quotes.
Macros are recognized in preference to the begin-quote string, so if
a prefix of START can be recognized as part of a potential macro name,
the quoting mechanism is effectively disabled. Unless you use
'changeword' (*note Changeword::), this means that START should not
begin with a letter, digit, or '_' (underscore). However, even though
quoted strings are not recognized, the quote characters can still be
discerned in macro expansion and in trace output.
define(`echo', `$@')
=>
define(`hi', `HI')
=>
changequote(`q', `Q')
=>
q hi Q hi
=>q HI Q HI
echo(hi)
=>qHIQ
changequote
=>
changequote(`-', `EOF')
=>
- hi EOF hi
=> hi HI
changequote
=>
changequote(`1', `2')
=>
hi1hi2
=>hi1hi2
hi 1hi2
=>HI hi
Quotes are recognized in preference to argument collection. In
particular, if START is a single '(', then argument collection is
effectively disabled. For portability with other implementations, it is
a good idea to avoid '(', ',', and ')' as the first character in START.
define(`echo', `$#:$@:')
=>
define(`hi', `HI')
=>
changequote(`(',`)')
=>
echo(hi)
=>0::hi
changequote
=>
changequote(`((', `))')
=>
echo(hi)
=>1:HI:
echo((hi))
=>0::hi
changequote
=>
changequote(`,', `)')
=>
echo(hi,hi)bye)
=>1:HIhibye:
However, if you are not worried about portability, using '(' and ')'
as quoting characters has an interesting property--you can use it to
compute a quoted string containing the expansion of any quoted text, as
long as the expansion results in both balanced quotes and balanced
parentheses. The trick is realizing 'expand' uses '$1' unquoted, to
trigger its expansion using the normal quoting characters, but uses
extra parentheses to group unquoted commas that occur in the expansion
without consuming whitespace following those commas. Then '_expand'
uses 'changequote' to convert the extra parentheses back into quoting
characters. Note that it takes two more 'changequote' invocations to
restore the original quotes. Contrast the behavior on whitespace when
using '$*', via 'quote', to attempt the same task.
changequote(`[', `]')dnl
define([a], [1, (b)])dnl
define([b], [2])dnl
define([quote], [[$*]])dnl
define([expand], [_$0(($1))])dnl
define([_expand],
[changequote([(], [)])$1changequote`'changequote(`[', `]')])dnl
expand([a, a, [a, a], [[a, a]]])
=>1, (2), 1, (2), a, a, [a, a]
quote(a, a, [a, a], [[a, a]])
=>1,(2),1,(2),a, a,[a, a]
If END is a prefix of START, the end-quote will be recognized in
preference to a nested begin-quote. In particular, changing the quotes
to have the same string for START and END disables nesting of quotes.
When quote nesting is disabled, it is impossible to double-quote strings
across macro expansions, so using the same string is not done very
often.
define(`hi', `HI')
=>
changequote(`""', `"')
=>
""hi"""hi"
=>hihi
""hi" ""hi"
=>hi hi
""hi"" "hi"
=>hi" "HI"
changequote
=>
`hi`hi'hi'
=>hi`hi'hi
changequote(`"', `"')
=>
"hi"hi"hi"
=>hiHIhi
It is an error if the end of file occurs within a quoted string.
`hello world'
=>hello world
`dangling quote
^D
error->m4:stdin:2: ERROR: end of file in string
ifelse(`dangling quote
^D
error->m4:stdin:1: ERROR: end of file in string

File: m4.info, Node: Changecom, Next: Changeword, Prev: Changequote, Up: Input Control
8.3 Changing the comment delimiters
===================================
The default comment delimiters can be changed with the builtin macro
'changecom':
-- Builtin: changecom ([START]
This sets START as the new begin-comment delimiter and END as the
new end-comment delimiter. If both arguments are missing, or START
is void, then comments are disabled. Otherwise, if END is missing
or void, the default end-comment delimiter of newline is used. The
comment delimiters can be of any length.
The expansion of 'changecom' is void.
define(`comment', `COMMENT')
=>
# A normal comment
=># A normal comment
changecom(`/*', `*/')
=>
# Not a comment anymore
=># Not a COMMENT anymore
But: /* this is a comment now */ while this is not a comment
=>But: /* this is a comment now */ while this is not a COMMENT
Note how comments are copied to the output, much as if they were
quoted strings. If you want the text inside a comment expanded, quote
the begin-comment delimiter.
Calling 'changecom' without any arguments, or with START as the empty
string, will effectively disable the commenting mechanism. To restore
the original comment start of '#', you must explicitly ask for it. If
START is not empty, then an empty END will use the default end-comment
delimiter of newline, as otherwise, it would be impossible to end a
comment. However, this is not portable, as some other 'm4'
implementations preserve the previous non-empty delimiters instead.
define(`comment', `COMMENT')
=>
changecom
=>
# Not a comment anymore
=># Not a COMMENT anymore
changecom(`#', `')
=>
# comment again
=># comment again
The comment strings can safely contain eight-bit characters. If no
single character is appropriate, START and END can be of any length.
Other implementations cap the delimiter length to five characters, but
GNU has no inherent limit.
Comments are recognized in preference to macros. However, this is
not compatible with other implementations, where macros and even quoting
takes precedence over comments, so it may change in a future release.
For portability, this means that START should not begin with a letter,
digit, or '_' (underscore), and that neither the start-quote nor the
start-comment string should be a prefix of the other.
define(`hi', `HI')
=>
define(`hi1hi2', `hello')
=>
changecom(`q', `Q')
=>
q hi Q hi
=>q hi Q HI
changecom(`1', `2')
=>
hi1hi2
=>hello
hi 1hi2
=>HI 1hi2
Comments are recognized in preference to argument collection. In
particular, if START is a single '(', then argument collection is
effectively disabled. For portability with other implementations, it is
a good idea to avoid '(', ',', and ')' as the first character in START.
define(`echo', `$#:$*:$@:')
=>
define(`hi', `HI')
=>
changecom(`(',`)')
=>
echo(hi)
=>0:::(hi)
changecom
=>
changecom(`((', `))')
=>
echo(hi)
=>1:HI:HI:
echo((hi))
=>0:::((hi))
changecom(`,', `)')
=>
echo(hi,hi)bye)
=>1:HI,hi)bye:HI,hi)bye:
changecom
=>
echo(hi,`,`'hi',hi)
=>3:HI,,HI,HI:HI,,`'hi,HI:
echo(hi,`,`'hi',hi`'changecom(`,,', `hi'))
=>3:HI,,`'hi,HI:HI,,`'hi,HI:
It is an error if the end of file occurs within a comment.
changecom(`/*', `*/')
=>
/*dangling comment
^D
error->m4:stdin:2: ERROR: end of file in comment

File: m4.info, Node: Changeword, Next: M4wrap, Prev: Changecom, Up: Input Control
8.4 Changing the lexical structure of words
===========================================
The macro 'changeword' and all associated functionality is
experimental. It is only available if the '--enable-changeword'
option was given to 'configure', at GNU 'm4' installation time.
The functionality will go away in the future, to be replaced by
other new features that are more efficient at providing the same
capabilities. _Do not rely on it_. Please direct your comments
about it the same way you would do for bugs.
A file being processed by 'm4' is split into quoted strings, words
(potential macro names) and simple tokens (any other single character).
Initially a word is defined by the following regular expression:
[_a-zA-Z][_a-zA-Z0-9]*
Using 'changeword', you can change this regular expression:
-- Optional builtin: changeword (REGEX)
Changes the regular expression for recognizing macro names to be
REGEX. If REGEX is empty, use '[_a-zA-Z][_a-zA-Z0-9]*'. REGEX
must obey the constraint that every prefix of the desired final
pattern is also accepted by the regular expression. If REGEX
contains grouping parentheses, the macro invoked is the portion
that matched the first group, rather than the entire matching
string.
The expansion of 'changeword' is void. The macro 'changeword' is
recognized only with parameters.
Relaxing the lexical rules of 'm4' might be useful (for example) if
you wanted to apply translations to a file of numbers:
ifdef(`changeword', `', `errprint(` skipping: no changeword support
')m4exit(`77')')dnl
changeword(`[_a-zA-Z0-9]+')
=>
define(`1', `0')1
=>0
Tightening the lexical rules is less useful, because it will
generally make some of the builtins unavailable. You could use it to
prevent accidental call of builtins, for example:
ifdef(`changeword', `', `errprint(` skipping: no changeword support
')m4exit(`77')')dnl
define(`_indir', defn(`indir'))
=>
changeword(`_[_a-zA-Z0-9]*')
=>
esyscmd(`foo')
=>esyscmd(foo)
_indir(`esyscmd', `echo hi')
=>hi
=>
Because 'm4' constructs its words a character at a time, there is a
restriction on the regular expressions that may be passed to
'changeword'. This is that if your regular expression accepts 'foo', it
must also accept 'f' and 'fo'.
ifdef(`changeword', `', `errprint(` skipping: no changeword support
')m4exit(`77')')dnl
define(`foo
', `bar
')
=>
dnl This example wants to recognize changeword, dnl, and `foo\n'.
dnl First, we check that our regexp will match.
regexp(`changeword', `[cd][a-z]*\|foo[
]')
=>0
regexp(`foo
', `[cd][a-z]*\|foo[
]')
=>0
regexp(`f', `[cd][a-z]*\|foo[
]')
=>-1
foo
=>foo
changeword(`[cd][a-z]*\|foo[
]')
=>
dnl Even though `foo\n' matches, we forgot to allow `f'.
foo
=>foo
changeword(`[cd][a-z]*\|fo*[
]?')
=>
dnl Now we can call `foo\n'.
foo
=>bar
'changeword' has another function. If the regular expression
supplied contains any grouped subexpressions, then text outside the
first of these is discarded before symbol lookup. So:
ifdef(`changeword', `', `errprint(` skipping: no changeword support
')m4exit(`77')')dnl
ifdef(`__unix__', ,
`errprint(` skipping: syscmd does not have unix semantics
')m4exit(`77')')dnl
changecom(`/*', `*/')dnl
define(`foo', `bar')dnl
changeword(`#\([_a-zA-Z0-9]*\)')
=>
#esyscmd(`echo foo \#foo')
=>foo bar
=>
'm4' now requires a '#' mark at the beginning of every macro
invocation, so one can use 'm4' to preprocess plain text without losing
various words like 'divert'.
In 'm4', macro substitution is based on text, while in TeX, it is
based on tokens. 'changeword' can throw this difference into relief.
For example, here is the same idea represented in TeX and 'm4'. First,
the TeX version:
\def\a{\message{Hello}}
\catcode`\@=0
\catcode`\\=12
@a
@bye
=>Hello
Then, the 'm4' version:
ifdef(`changeword', `', `errprint(` skipping: no changeword support
')m4exit(`77')')dnl
define(`a', `errprint(`Hello')')dnl
changeword(`@\([_a-zA-Z0-9]*\)')
=>
@a
=>errprint(Hello)
In the TeX example, the first line defines a macro 'a' to print the
message 'Hello'. The second line defines <@> to be usable instead of
<\> as an escape character. The third line defines <\> to be a normal
printing character, not an escape. The fourth line invokes the macro
'a'. So, when TeX is run on this file, it displays the message 'Hello'.
When the 'm4' example is passed through 'm4', it outputs
'errprint(Hello)'. The reason for this is that TeX does lexical
analysis of macro definition when the macro is _defined_. 'm4' just
stores the text, postponing the lexical analysis until the macro is
_used_.
You should note that using 'changeword' will slow 'm4' down by a
factor of about seven, once it is changed to something other than the
default regular expression. You can invoke 'changeword' with the empty
string to restore the default word definition, and regain the parsing
speed.

File: m4.info, Node: M4wrap, Prev: Changeword, Up: Input Control
8.5 Saving text until end of input
==================================
It is possible to 'save' some text until the end of the normal input has
been seen. Text can be saved, to be read again by 'm4' when the normal
input has been exhausted. This feature is normally used to initiate
cleanup actions before normal exit, e.g., deleting temporary files.
To save input text, use the builtin 'm4wrap':
-- Builtin: m4wrap (STRING, ...)
Stores STRING in a safe place, to be reread when end of input is
reached. As a GNU extension, additional arguments are concatenated
with a space to the STRING.
The expansion of 'm4wrap' is void. The macro 'm4wrap' is
recognized only with parameters.
define(`cleanup', `This is the `cleanup' action.
')
=>
m4wrap(`cleanup')
=>
This is the first and last normal input line.
=>This is the first and last normal input line.
^D
=>This is the cleanup action.
The saved input is only reread when the end of normal input is seen,
and not if 'm4exit' is used to exit 'm4'.
It is safe to call 'm4wrap' from saved text, but then the order in
which the saved text is reread is undefined. If 'm4wrap' is not used
recursively, the saved pieces of text are reread in the opposite order
in which they were saved (LIFO--last in, first out). However, this
behavior is likely to change in a future release, to match POSIX, so you
should not depend on this order.
It is possible to emulate POSIX behavior even with older versions of
GNU M4 by including the file 'm4-1.4.17/examples/wrapfifo.m4' from the
distribution:
$ m4 -I examples
undivert(`wrapfifo.m4')dnl
=>dnl Redefine m4wrap to have FIFO semantics.
=>define(`_m4wrap_level', `0')dnl
=>define(`m4wrap',
=>`ifdef(`m4wrap'_m4wrap_level,
=> `define(`m4wrap'_m4wrap_level,
=> defn(`m4wrap'_m4wrap_level)`$1')',
=> `builtin(`m4wrap', `define(`_m4wrap_level',
=> incr(_m4wrap_level))dnl
=>m4wrap'_m4wrap_level)dnl
=>define(`m4wrap'_m4wrap_level, `$1')')')dnl
include(`wrapfifo.m4')
=>
m4wrap(`a`'m4wrap(`c
', `d')')m4wrap(`b')
=>
^D
=>abc
It is likewise possible to emulate LIFO behavior without resorting to
the GNU M4 extension of 'builtin', by including the file
'm4-1.4.17/examples/wraplifo.m4' from the distribution. (Unfortunately,
both examples shown here share some subtle bugs. See if you can find
and correct them; or *note Answers: Improved m4wrap.).
$ m4 -I examples
undivert(`wraplifo.m4')dnl
=>dnl Redefine m4wrap to have LIFO semantics.
=>define(`_m4wrap_level', `0')dnl
=>define(`_m4wrap', defn(`m4wrap'))dnl
=>define(`m4wrap',
=>`ifdef(`m4wrap'_m4wrap_level,
=> `define(`m4wrap'_m4wrap_level,
=> `$1'defn(`m4wrap'_m4wrap_level))',
=> `_m4wrap(`define(`_m4wrap_level', incr(_m4wrap_level))dnl
=>m4wrap'_m4wrap_level)dnl
=>define(`m4wrap'_m4wrap_level, `$1')')')dnl
include(`wraplifo.m4')
=>
m4wrap(`a`'m4wrap(`c
', `d')')m4wrap(`b')
=>
^D
=>bac
Here is an example of implementing a factorial function using
'm4wrap':
define(`f', `ifelse(`$1', `0', `Answer: 0!=1
', eval(`$1>1'), `0', `Answer: $2$1=eval(`$2$1')
', `m4wrap(`f(decr(`$1'), `$2$1*')')')')
=>
f(`10')
=>
^D
=>Answer: 10*9*8*7*6*5*4*3*2*1=3628800
Invocations of 'm4wrap' at the same recursion level are concatenated
and rescanned as usual:
define(`aa', `AA
')
=>
m4wrap(`a')m4wrap(`a')
=>
^D
=>AA
however, the transition between recursion levels behaves like an end of
file condition between two input files.
m4wrap(`m4wrap(`)')len(abc')
=>
^D
error->m4:stdin:1: ERROR: end of file in argument list

File: m4.info, Node: File Inclusion, Next: Diversions, Prev: Input Control, Up: Top
9 File inclusion
****************
'm4' allows you to include named files at any point in the input.
* Menu:
* Include:: Including named files
* Search Path:: Searching for include files

File: m4.info, Node: Include, Next: Search Path, Up: File Inclusion
9.1 Including named files
=========================
There are two builtin macros in 'm4' for including files:
-- Builtin: include (FILE)
-- Builtin: sinclude (FILE)
Both macros cause the file named FILE to be read by 'm4'. When the
end of the file is reached, input is resumed from the previous
input file.
The expansion of 'include' and 'sinclude' is therefore the contents
of FILE.
If FILE does not exist, is a directory, or cannot otherwise be
read, the expansion is void, and 'include' will fail with an error
while 'sinclude' is silent. The empty string counts as a file that
does not exist.
The macros 'include' and 'sinclude' are recognized only with
parameters.
include(`none')
error->m4:stdin:1: cannot open `none': No such file or directory
=>
include()
error->m4:stdin:2: cannot open `': No such file or directory
=>
sinclude(`none')
=>
sinclude()
=>
The rest of this section assumes that 'm4' is invoked with the '-I'
option (*note Invoking m4: Preprocessor features.) pointing to the
'm4-1.4.17/examples' directory shipped as part of the GNU 'm4' package.
The file 'm4-1.4.17/examples/incl.m4' in the distribution contains the
lines:
$ cat examples/incl.m4
=>Include file start
=>foo
=>Include file end
Normally file inclusion is used to insert the contents of a file into
the input stream. The contents of the file will be read by 'm4' and
macro calls in the file will be expanded:
$ m4 -I examples
define(`foo', `FOO')
=>
include(`incl.m4')
=>Include file start
=>FOO
=>Include file end
=>
The fact that 'include' and 'sinclude' expand to the contents of the
file can be used to define macros that operate on entire files. Here is
an example, which defines 'bar' to expand to the contents of 'incl.m4':
$ m4 -I examples
define(`bar', include(`incl.m4'))
=>
This is `bar': >>bar<<
=>This is bar: >>Include file start
=>foo
=>Include file end
=><<
This use of 'include' is not trivial, though, as files can contain
quotes, commas, and parentheses, which can interfere with the way the
'm4' parser works. GNU 'm4' seamlessly concatenates the file contents
with the next character, even if the included file ended in the middle
of a comment, string, or macro call. These conditions are only treated
as end of file errors if specified as input files on the command line.
In GNU 'm4', an alternative method of reading files is using
'undivert' (*note Undivert::) on a named file.

File: m4.info, Node: Search Path, Prev: Include, Up: File Inclusion
9.2 Searching for include files
===============================
GNU 'm4' allows included files to be found in other directories than the
current working directory.
If the '--prepend-include' or '-B' command-line option was provided
(*note Invoking m4: Preprocessor features.), those directories are
searched first, in reverse order that those options were listed on the
command line. Then 'm4' looks in the current working directory. Next
comes the directories specified with the '--include' or '-I' option, in
the order found on the command line. Finally, if the 'M4PATH'
environment variable is set, it is expected to contain a colon-separated
list of directories, which will be searched in order.
If the automatic search for include-files causes trouble, the 'p'
debug flag (*note Debug Levels::) can help isolate the problem.

File: m4.info, Node: Diversions, Next: Text handling, Prev: File Inclusion, Up: Top
10 Diverting and undiverting output
***********************************
Diversions are a way of temporarily saving output. The output of 'm4'
can at any time be diverted to a temporary file, and be reinserted into
the output stream, "undiverted", again at a later time.
Numbered diversions are counted from 0 upwards, diversion number 0
being the normal output stream. GNU 'm4' tries to keep diversions in
memory. However, there is a limit to the overall memory usable by all
diversions taken together (5